How do I change the T-Mobile banner on my Samsung Gravity T?

I recently got a Samsung Gravity T from T-Mobile and the banner on it on all the screens says "T-Mobile." I know you can change this… or at least you should be able to. But I've searched all over my phone and I can't find how to change it. I looked in "display setting" and it only had a "Greetings Message," but that only shows up when you first turn the phone on.

Does anyone have a Gravity T and know how to change the "T-Mobile" banner?
Your help would be much appreciated! =)