You can change them – folders, apps or whatever – manually by...
1. Select the icon you want to change your folder, app or file to.
2. Choose "Get Info" from the File Menu, or click command-I.
3. Click the icon in the upper left of the info window.
4. Choose "Copy" from the Edit menu (or type command-C).
5. Select your folder, app or file on the desktop and "Get Info" for it.
6. Click the icon in the upper left of the info window.
7. Select "Paste" (command-V) from the Edit Menu.
And that should do the trick. There are also apps like
CandyBar which you can use for system wide changes and for icons like Trash and the Finder which you can't alter without some behind the scenes fiddling.