After your initial username modification you are just allowed to alter it as soon as, according to the guidelines and standards of Facebook.
How To Get Username On Facebook
Get a New URL.
Action. 1 Log in to Facebook.
Action. 2 Clear the URL bar, and key in "www.facebook.com/username." Select from the list of offered names, or
inspect package beside the empty field to enter a custom username.
Action. 3 Click the "Examine Availability" button, and click the "Confirm" button to finish the process.
Modification Your Existing URL.
Action. 1 Click the "Settings" button at the top of the Facebook site.
Action. 2 Click the "Change" link beside the "Username" section. Highlight your existing user name and press the "Erase" or
"Backspace" button to remove the existing name, and type in a new username.
Action. 3 Click the "Examine Availability" button, and click "Verify" to finish the switch How To Get Username On Facebook.