You need to copy the icon (in Finder, select the icon and click the Apple key and "C"), then in Finder, go to the icon you want to replace, select it then hit Apple and "I" (to get the info screen). This will open a new window where you will see the old icon in the top left corner. Select this and it should get like a blue hue around it then hit paste (Apple and "V") which should replace the old one with the new one.