“Get Info” (Command+I) on the file with an icon you want to change.
Drag and drop the file with the replacement icon directly onto the Get Info icon well .
“Get Info” (Command+I) on the file with the icon you want to use. Then click on the little icon near the top-left corner of the Get Info window. You’ll noticed the icon has a faint blue outline after you click on it. Now copy the icon by pressing Command+C.
Repeat the above for the file with its icon you want to replace, but Paste (Command+V) the copied icon instead.
Restore Default Icon
To remove a custom icon, just click the Get Info’s icon well and press the Delete button on your keyboard.
You can’t change an icon if you do not have write permission to the file.