Facebook Fan Pages there is a limit of minimum 25 fans to qualify for a custom username as their Fan Page slug. Also, once allowed you cannot change the username of your page, it is done to keep the quality of service.
Welcome simjo, ok i will help you, Go to Account settings page and u will see the 2nd line is user name you can change there with your custom name. Choose your new username carefully. You can only change your username once.