Its very simple. I will teach you in 3 easy steps on how to create or change the username of a facebook page.
Whenever you are choosing a username make sure you are choosing the username which you really like. Because changing a username can be slightly hard. As its not appreciated by facebook. So you cannot change username every second day.
So, lets start.
First from your admin account login and goto about section
then on next screen select create username
and then it will bring you to a new screen where it would ask you to choose the username you would like.
So simple choose the username and there you are. You have successfully chosen a new username for your facebook page. Congratulations