After your initial username change you are only allowed to change it when, according to the rules and standards of Facebook.
What's My Facebook Username
Get a New URL.
Action. 1 Log in to Facebook.
Action. 2 Clear the URL bar, and enter "www.facebook.com/username." Select from the list of offered names, or check package next to the empty field to go into a custom-made username.
Action. 3 Click the "Inspect Availability" button, and click the "Confirm" button to finish the process.
Change 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" area. Highlight your existing user name and press the "Delete" or
"Backspace" button to eliminate the existing name, and enter a new username.
Action. 3 Click the "Check Availability" button, and click "Validate" to finish the switch What's My Facebook Username.