Select the file, and do a Get Info on it. Click on the icon image in the Get Info window and enter CMD-C (copies it). Now click on the application in question, and do a Get Info on it. Click on the icon image in its Get Info window and type CMD-v. This pastes your previously copied icon. Done!
The apps icon is an .icns file, the same as the replacement icon that you want to use.
This is a tricky procedure but not impossible. Follow the instructions from this link. (Clicky)
There is a warning with this type of modding.
Warning: Before you modify or replace the ICNS make sure to create a backup of the application you're modifying. If you follow these instructions, you do so at your own risk.
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