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 Bob's your mothers brother. 🙂