You mean editing skins?
Go to Warcraft 3 Viewer, find a model in the Treeview, extract it and convert the .blp file (that's the texture). After you've converted the .blp file with Wc3 Viewer (BLP > TGA, BMP, JPG convert button) you can edit the skin/texture.
Importing option A:
And if you want your new texture to be used in the designated model (ex. a Footman with a new golden armour skin), you go to Magos' Model Editor, open the .mdx file, go to Windows>Texture Manager, remove the old present-skin and import the new skin. This may result in the Editor that it'll look black, but nothing to worry about, just Save. Now go to World Editor and import the .mdx and the .blp file.
Importing option B:
Just go to World Editor, find the Footman's path (it's human/melee/units/footman right?) and remember the path and/or copy it. Now import your new .blp file in Import Manager, go to the imported file's path (where ''/war3imported/...'' is) and paste your path over it.
Am I close? Someone tell me if I did something wrong in it.