This is hopefully a simple way of changing textures without digging too deeply into it. A fairly basic-intermediate understanding of Photoshop is needed.
If you open the texture file you’ll see something like this.
Save another version with a new filename. I tend to work with Tiff to retain layers, though I’m sure somebody will tell me why Targa is a better filetype.
If you open up your reference file of any of the characters, go into the Window > Rendering Editors > Hypershade and go into Textures. You should see anim_color_MAP(file) (this may be a different name for other characters). Click on that and in the attribute editor you should see Image Name under File Attributes. Click the small folder, find your new texture map and load that in.
In the texture map each area corresponds to a different area of the body. If I change it to simple colours you should be able to see what area of the texture map applies to which part of the rig. The red area is the right arm, the green is the torso, etc.
A good way to get a general guide of where things will be on the texture map is to use the 3D Paint Tool. You can basically roughly paint on the mesh and then clean it up in Photoshop. Select your mesh first then, go into your Rendering shelf click the 3D Paint Tool icon (should be the last one). Next, in the Attribute Editor click Assign/Edit Textures. Now you should be able to freely paint on the mesh.
When you’ve got a rough idea painted, hit the Save Textures button in the Attribute Editor. In your project’s Source Images folder, you should have a new 3DPaintTextures folder with the the new texture map inside. Using this you can now make a cleaner final version in Photoshop.