I think your username is associated with when you first opened a account and it's associated with your password...
You could try changing your password and see if that option also comes up..
If you don't see the change username option when you try then leave it alone..
Or, you could just open a new account if it lets you..
Just keep in mind, these are just suggestions so if things go wrong don't blame me..
Better yet, email OBS support and ask them.