How to Change App icon in Mac
In Mac OS, all the applications have .app extension but it ships in the form of a zip file. So you can easily check the contents of any application.
The application icon is actually an icon file with .icns extension stored in the application. To change the icon of application, you need to find out the existing icon file and then replace it with the desired one. For example, here I am changing Firefox icon to some other icon.