CandyBar is a good program for this...try it for free, but it's $12.95 to buy.
If you only want to change a couple of icons manually, you can copy the icon you want to the clipboard, open "Get Info" on the file/drive you want to change, click on the icon in the Info window, and paste the clipboard contents.
You can just find the image you want and copy it, then go into the "Get Info" for the item you want to change, click on the icon (it will highlight) and command-v to paste... should be as simple as that.