After your initial username modification you are only permitted to alter it when, according to the guidelines and standards of Facebook.
How To See Facebook Username
Get a New URL.
Action. 1 Visit to Facebook.
Action. 2 Clear the URL bar, and enter "www.facebook.com/username." Select from the list of available names, or check the box next to the empty field to enter a customized username.
Action. 3 Click the "Check Availability" button, and click the "Confirm" button to finish 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 next to the "Username" section. Highlight your existing user name and press the "Erase" or
"Backspace" button to erase the existing name, and key in a brand-new username.
Action. 3 Click the "Examine Schedule" button, and click "Confirm" to complete the switch How To See Facebook Username.