Step 1: Download InfranView (you can get it by searching infranview on google) and install it.
Step 2: Download paint.NET (you need this to edit the file without the textures going wrong), use same method as step 1
Step 3: Go to your NZP directory, Then click on the file with all the data and open progs folder.
Step 4: You will notice that there will be files which are .pcx, these are the texture files for the model.
Right click and go to open with... and click browse (if not an animal wearing a mask there) and find the directory where you installed it in.
Step 5: You have opened it, click open on the program and find you the file you want to retexture. Then go to file and save as, and save it as a bitmap.
Step 6: Edit the bitmap with paint.NET. To save you must save it as a bitmap with 8 bit detail (auto detect will not work)
Step 7: Go back to InfranView and open the file, then save it as a .pcx file. Save it in progs and now to have made your texture.
Comment for improvements to tutorial or to say if it was helpful or not.
P.S. A good program to edit .mdl's is Quake Army Knife