After your preliminary username modification you are only allowed to alter it when, according to the rules and standards of Facebook.
How To Get My Facebook Username
Get a New URL.
Action. 1 Visit to Facebook.
Action. 2 Clear the URL bar, and type in "www.facebook.com/username." Select from the list of available names, or
inspect the box beside the empty field to get in a custom-made username.
Action. 3 Click the "Examine Schedule" button, and click the "Verify" button to complete the process.
Modification Your Existing URL.
Action. 1 Click the "Settings" button at the top of the Facebook website.
Action. 2 Click the "Change" link next to the "Username" area. Highlight your existing user name and press the "Erase" or
"Backspace" button to eliminate the existing name, and type in a new username.
Action. 3 Click the "Examine Schedule" button, and click "Confirm" to complete the switch How To Get My Facebook Username.