change profile page name

please help me how i can change my facebook page name. when i go update my page . Then show this picture below

 

Go to facebook page ->click edit page drop down -> click update public info -> click on create a username for this page -> click continue -> enter a desired name.
After you set your username, you may only change it once.

 

please see this picture here this option not found .

 

By the way! This depends a lot on the number of your page followers. There's a specific number of followers that, once passed, you can't change the page name no more.