Do I have to re-register a new Facebook account to change my facebook.com/USERNAME?Or are there any ways to change it?
How to change Facebook username?
facebook;account management
You can only change your Facebook username once. If you have already changed it then there is no way to change it again. You would have to setup a new account and claim a new username that way.The option to change your username is in Account Settings > Settings > Username. If that option is not available then you cannot change your username.Taken from this Facebook support article.How do I change my username?You can change your username from the Settings tab of the Account  Settings page. Scroll down to Username and click change. Then  follow the on-screen directions. Keep in mind you will only be able to  change your username once.