I haven't used 3.9 in awhile, but I believe that if you open an information requester for the icon you wish to change you can then drag the icon you wish to replace it with into the requester (and drop over the icon preview in the top left hand corner) it will change the icon.
Failing that there are utilities on Aminet that will let you change icons quite easily. I used one for years, but I can't remember what it is called.
