Hey! As per request, I will now give a bit of an intro on how to adjust what parts of a texture are mapped what parts of the model.
There likely are more complete ways, for example using Blender, or 3DS Max which natively supports .nif files, however so far in my experience Blender has its issues dealing with Guild 2 .nifs even with plugins, and 3DS Max is a very pricey piece of software to use for modding, so I'm sticking to a simpler tool: NifSkope.
If you need NifSkope, you can get it here: http://niftools.sourceforge.net/wiki/NifSkope
Upon launching NifSkope, you can open up a .nif file using File>Open in the upper left, after which you should see a similar sight to what you see below, minus the two smaller square windows (and arrows, naturally).
Below the image, I'll go through the relevant parts of the software point by point, using the numbered arrows to guide you.
This example will be using the (Vanilla) thief_f.nif file stored in Objects/Characters, as well as the corresponding cm_thief_f_body_01.dds which is stored in the Textures folder.
1 - This button isn't necessary to use, however it might be visually helpful. Guild 2 models for the most part come in multiple LOD-s (Levels of Detail), which the game switches between invisibly as the camera gets closer or further from characters. This button simply governs whether NifSkope will mimic this behavior or not. When it is off like in the image, only one LOD is rendered (though all are accessible on the left, we'll get to it). It might be useful to turn it on, if you want to see which LOD you selected on the left represented visually.
2 - Similar to 1, this is optional, in fact, very much irrelevant here, except for an important note if you ever wish to adjust the models in other ways with NifSkope. That button governs whether or not the model is in its default T-Pose, or if it's in the default pose of the animation rig assigned to it. If you plan to do any adjustments to the actual model itself, make sure it looks right with that button turned on, as sometimes, while fine in T-Pose, the model may give surprising results in actual animation.
Moving onto the relevant stuff:
3 - As mentioned before, most character models and also some object models in Guild 2 come in multiple LODs. On the left there, is where the .nif's data "blocks" are listed, and of special interest are these LOD NiNodes.
If you plan to adjust the texturing on a model, keep in mind that you'll usually need to repeat the same adjustments on multiple LODs. However, some LODs are so rough around the edges, that they barely qualify as stick figures, being intended to be the view of objects/Characters from halfway across the map, so using the button described in point (1) may give you a good idea whether or not a given model is really worth the effort, or if it's best to just leave it alone.
You do not actually need to do anything with these NiNodes, but keep an eye out for NiNodes labeled LOD[number], to know in advance whether you'll need to repeat the process.
4 - The actual parts of the .nif file you want to alter are these NiTriShapes. These are essentially what contains most of the actual model data in the .nif, with the rest taking up room for animation rigs, various flags used by the game to know what's what, etc.
For characters, each LOD likely comes with 3 NiTriShapes, or 4 if it's a female model. Sometimes it can be more, sometimes it can be less. If a character wears a Helm/Hat or Hair, that is stored in a separate .nif called hairlibrary.nif for hairs, or hatlibrary.nif of helms/hats. Both of those are in Objects/Characters.
The 3+1 usual NiTriShapes correspond to the body, the head, the eyes and in case of female models: The eyelashes.
You probably only want to edit the body or the head. You can edit only that, it won't mess with the other 2 or 3 parts.
Right clicking one of these NiTriShapes brings up a couple of options. (You can also right click the rendered model in the main window, but it's advisable to click in the block list on the left, so you know for sure you clicked the correct thing. Sometimes, especially if multiple LOD-s are displayed at once, it's easy to get confused.)
The option you need to re-adjust the texture map is "Texture>Edit UV". This will bring up the window pointed out in the next point.
5 - This window, is the UV editor. Ignore the second version of the window n the left for now. Before I say anything about it, know that it has a bad habit of closing as soon as you click anywhere outside of it, and this includes clicking on NifSkope's icon in the system tray, if you switched to a different window. If you are using NifSkope while also looking at something else (either the texture, this guide or anything else), a quick work around is to have NifSkope on one side of your screen, and whatever else you're looking at on another. The UV Editor won't close until you click on something inside NifSkope itself, other than the UV Editor.
The green nets of green lines displayed inside it, are the triangles that make up the surface of the model you can see in the back view, cut up into sets. One word I want you to remember, as I will use it, is "vertex". This is essentially just the name for the corners of the triangles. You can click each vertex individually, and then if you click them again and hold, you can move them around. However, you probably want to move them in a more organized fashion, so here's a more in-depth look on this window:
The bright gray square, where in this model you can see most of the green nets laid out, is essentially where you should imagine the texture is also located. So, if you look at point (6) which is the actual cm_thief_f_body_01.dds texture, if you mentally move it where the bright gray square is, you can start to see which triangles use what part of the texture to find out what they're supposed to show in-game.
So, for example the large net of green triangles, which is close to the upper left corner of the bright gray area, just next to what looks like a column or a ruler, is entirely above the white, folded cloth looking texture in the upper left of cm_thief_f_body_01.dds. As a matter of fact, that set of triangles, are the ones that make up the arms of the female thief character.
It should be noted, that the green triangles outside of the bright gray area aren't there by mistake, and in fact, you should always zoom out when editing a new object in this view, to see if there is any of them outside of the bright gray area.
As a matter of fact, the only reason they are out of there, is to easily distinguish between parts of the model that is assigned to the same part of the texture. Just as you imagined moving the texture (again, the one pointed to by arrow (6)) to be over the light gray square, you should imagine, that the same texture repeats over and over again in every direction.
Essentially, if you moved any of these triangles or groups of triangles to the left or right, or up or down by exactly as much as is the size of the bright triangle, then it wouldn't change one bit what they look like in-game, as when the game actually loads in, it virtually moves the triangles outside of that rectangle into the bright gray square by moves sized equal to the square itself.
You can mimic the same thing, by clicking one of the vertexes, then right clicking it and selecting the option "Select Connected". This will select every vertex, and as a result, every triangle that is connected to the vertex you clicked. At this point, you can either click and drag them, or if you want to be precise, right click again and select "Scale and Translate Selected" (or just press Alt+S). This will allow you to scale and move all the vertexes you selected at once.
Scaling is either done uniformly (will increase/decrease horizontally and vertically by the same amount) or by un-ticking "Uniform scaling", separately for the horizontal X and vertical Y axis.
Moving (translation) takes movement on the X and Y axis as a variable. The direction is always "positive number push right and down, negative numbers push left and up".
To my experience, moving vertexes by multiples of 2 in either or both axis will result in having them move by one repeat of the texture. So essentially, if you move the triangles by that much, the part of the texture they are mapped to, doesn't change.
6 - This is simply me showing the texture in question. In reality, I had to open up the .dds fileinside GIMP to see it, as to my experience, NifSkope doesn't have simple feature to see textures for Guild 2, the game itself assigning the texture during runtime. I added it here for a visual aid, you should find a program to view .dds files with separately. GIMP and I believe Paint.net are both excellent free ways to do this, though both might need extra plugins.
As a bit of a viewing aid, I will now also insert a picture of the in-game female thief model.