To change file icons, you must do it manually through a key command (unless they are system file icons, but this I will explain in a later tutorial).
The command we will be using is Cmd + i.
With an Icon file.
Sometimes when you download a file or application, a folder containing an Icon file will be included in the download (empty folder with the icon as icon). If this is the case, consider yourself lucky.
If you want to use an icon you created yourself (in Photoshop etc.), you will need the application CandyBar. CandyBar makes the process easy; all you need to do is open your icon in CandyBar, right click it, and select “Export” to Icon.
Then select your Icon file and press Cmd + i (+i).
A window of this type will appear. Simply select the icon in the top left corner and copy it (Cmd + c).
Now, navigate to the file, folder, or application that you would like to replace the icon of. Press Cmd + i again, and the info window will appear again. Select the top left icon and paste (Cmd + v).
Without CandyBar or an Icon file.
If the file or the application you downloaded did not come with an Icon, or you do not have CandyBar. We can use the icns or png file.
Open your downloaded icon file in Preview and go to Edit > Copy. Navigate to the file, folder, or application that you would like to replace the icon of and select it. Press Cmd + i. The info window will appear. Select the icon at top left and paste the image.