How to change my personal username on facebook?

[Alexandra]

Hello I don't have access to a computer at the moment but I need to know if there is any way on the Facebook mobile app that I can change my personal username/facebook page URL which is still under my old name? I read the guide on Facebook but I can't find the setting anywhere on the mobile site or IPhone app, when I go to account settings it will only say "name" with no setting for "username" and my facebook email is also still under my old name and I have no way to change it Help me!
I already changed my name but my USERNAME and facebook email is still under my old name!