Select the icon you want, Get Info (cmd-I), click on the icon in the upper left corner of the Get Info window, copy (cmd-c) and close the window.
Select the icon you want to change, Get Info (cmd-I), click the icon in the upper left corner of the Get Info window, paste (cmd-P). That should be it.
If you want to change folders system wide, I'm not sure how to do that. Maybe someone else will.