An easy way of doing this is to-
1.Copy the icon that you want to use as the replacement.
2.Make sure that you complete step one before you continue!
3.Click once on the file or folder that you want to transfer the new icon to.
4.Press Command-I on your keyboard.
5.Click once on the icon On the top left of the Info window that pops up.
6.Press Command-X on your keyboard.
7.Press Command-V on your keyboard.
8.Enjoy your new icon.