The explination below just work for folders, files or applications icons, not for default system folder icons or Finder and Trash icons in the Dock.
To change the icon on folder, file or application you need to have write permissions. If you are not agree with the changes you can undo your action, bring up the "Get Info" window for the object you would like to undo change. Click on the icon in the window, select in the Edit menu "Cut" or press "Command-x". The new icon will be deleted and the ancient (the default icon) will reappears.