I'll assume you want to change the embedded icon in an application.
For example, if you wanted to change CCleaner's icon to something a bit nicer, you would navigate to %programfiles%\ccleaner
, open CCleaner.exe with Restorator.
In the left hand pane, expand Icon
, right click the icon you wish to change and choose Assign > Assign to...
Choose your prefered .ico image, save & close restorator.
Please note that sometimes the icons will not refresh straight away. Delete the thumbnail cache, and see here