download warcraft 3 viewer, open it. file -> open mpq -> ... if the icon you want to change is from the expansion, then choose war3x.mpq (in your warcraft 3 directory), else choose war3.mpq. press ctrl + H if the Treeview window isn't already open. in the treeview select the BLP radio button. expand ReplacableTextures. expand CommandButtons. search for the icon you want to change, right-click on it -> extract (and convert) BLP ... after saving it, you've got an icon to edit
if you have photoshop i can tell you how to make a button to look disabled or passive
please tell me what you've done so far, if it worked and so on, then i'll post how to import it back into warcraft.