After your preliminary username change you are only allowed to change it when, inning accordance with the rules and guidelines of Facebook.
How To Know Facebook Username
Get a New URL.
Action. 1 Visit to Facebook.
Action. 2 Clear the URL bar, and key in "www.facebook.com/username." Select from the list of readily available names, or check package beside the empty field to go into a custom username.
Action. 3 Click the "Check Accessibility" button, and click the "Validate" button to complete the procedure.
Modification Your Existing URL.
Action. 1 Click the "Settings" button at the top of the Facebook website.
Action. 2 Click the "Change" link beside the "Username" area. Highlight your existing user name and press the "Delete" or
"Backspace" button to eliminate the existing name, and enter a brand-new username.
Action. 3 Click the "Inspect Accessibility" button, and click "Validate" to complete the switch How To Know Facebook Username.