It’s fairly simply to change an app or folder icon in Mac OS X. The possibilities are endless. You could change an app icon to one that looks a little more polished in the dock. You could customize a folder you access often to make it easier to find. Whatever your reason may be, here’s a step-by-step guide on how to do it.
How to Change App and Folder Icons on Your Mac
Step 1:
Choose an image. You can find many alternative icons with a quick Google search, or on deviantart.com. A .icns format is preferred, since it contains multiple icon sizes in a single file for use across the system. A .png file will work as well, but you’ll want one that has a transparent background (not white). The recommended size is 512x512 pixels.
Step 2:
In Finder, select the application or folder whose icon you would like to change. Right-click the application and click Get Info (or use the shortcut command + I).
Step 3:
In the Info panel that pops up, you’ll see an icon at the top left corner. If your icon is a .icns file, you can just drag and drop the new icon onto the old one in the Info panel.
If it is a .png file, you might have to use the copy-and-paste method. Double-click the new icon to open it in the Preview application. Press command+A to select all, and command+C to copy. Click on the old icon in the Info panel to highlight it, and press command+V to paste. Voilà!
Step 4:
If OS X asks for your administrator password at this point, enter it. This permission is needed to change some icons.
Step 5:
Some icons (especially those in the dock) might not show up until you log out. To fix this you can use a quick Terminal command to “reset” the dock. Open Terminal (In Applications > Utilities) and type in “killall Dock” (without quotes) just like in the screenshot below. Press enter. If you are unfamiliar or uncomfortable with Terminal, skip this step and just log out and log back in. Terminal is a very powerful program that can harm your Mac if used improperly.
Note: Certain icons, such as Finder, Calendar, and Trash, cannot be changed using this method. To change these icons, use LiteIcon, or follow this guide.
It’s also possible to revert back to the original icon if you don’t like the new one. Simply go back to the Info panel (Step 2), click on the icon to highlight it, and press backspace (delete). Again, you may be prompted for your password.
That’s it! Enjoy your Mac's fresh new look.