Haha, I replaced mine yesterday. 🙂
I did it the more 'simple' way without using the terminal. If you're comfortable with the terminal however, I would recommend you using it but if you're not do it this way: At first you have to make a folder named after your icon. For example:
"Safari icon(s)". After you're done with that, Right or Ctrl click on your icon and then select "duplicate". Then just drag your copy to the folder and you're done. 🙂
Hope I helped. 🙂