Click About on the left side of your Page. Under General and Username, click Create Page @username or Edit. Enter a new Username and click Create Username.
Why can't I change my username on my Facebook page?
Keep in mind, you can only change your name every 60 days. Your name doesn't follow our name policy. You changed your name in the last 60 days, or you tried to change it too frequently. You were previously asked to confirm your name on Facebook.