You have sated your facebook username one time, but you can change it too. facebook allow two time change user name. If you have done two time, you can’t change 3rd time. So be careful during change your user name. 🙂
Just log on your facebook.com with your email id and password.
Then after go to Setting > Account Setting > Unsername
Here you can check your username available or not. If available you can chose this username which you want 🙂
Now your username has been set! 😀