Open folder with Mac icons in .ICNS format in Finder and copy icon (Cmd+C).

Select Application or Folder. Right-click to get Info (Cmd+I) and select the application icon in the top left corner of the window.

Paste new icon (Cmd+V).

To revert default icon, select icon in the top left corner of info window and delete it (Backspace).

See also how to change the new iTunes 10 icon. And our large free set of icons for iOS (and Windows 8 too). And OS X toolbars too. And for street navigation. And for designing a postcard for your grandma.