To change your profile’s username, go to Settings > Account Settings, and next to Username, click Change.
“Choose your new username carefully. You can only change your username once.” it warns.
Annoyingly, it does not allow you to change the formatting of your current username, such as the capitalization of letters or the location of dots. Because capital letters and dots do not make a username unique, when you click “Check Availability” it checks against all current usernames and reports that it’s not available because someone is already using it: you. Hopefully Facebook fixes this issue in the future.
Facebook suggests that when choosing a username you should “make sure it is something you like and that it’s appropriate for friends, family, co-workers, and anyone else who might use it to find you.”
Will you be changing your username?