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.