Changing the icon is simply a process of copying and pasting. Go to the application which has the icon you want to copy, right click and choose "Get Info" - now click on the icon at the top left corner:
You will notice that the icon is now highlighted in a light blue color. Press Cmd-C to copy the icon. Repeat the above process but this time on the application you want to change the icon, highlight the icon and press Cmd-V to paste the new icon.