Either we use this direct method which is more tedious, but the only effective if you only have an icns file and do not have candybar. Note that if you have a png file, you must convert it to .Icns. You can find out how to do this in the Mac Icon Creation tutorial, here.
This method applies only to change the icon of an application. And you need an icns.
To do this, find the application in your applications folder and then right-click it and select “Show Package Contents”.
After, a window will open. Go to Contents > Resources, then locate the application icon (which usually bears the name of the application) and name your file. Icns in the same way. Then replace the one located in “Resources” by yours.
Caution : It is always good to make a backup of the icon based on if you want to restore.
That is, if the application is in the dock, it is possible that it will have to be removed and put back in the dock in order for you to see the change.