UnJailbreak iPhone 4?

Hi, this is my first time here; I just signed up; and I was wondering how do I unjailbreak my iPhone 4 on iOS 5.1 WITHOUT losing any game data? Ex: I completed Cut the Rope and I don't want to lose all the levels I beat.

-Thanks


EDIT: I wasn't also sure where to post this question.

Back it up via iTunes, it should do this every time you sync it.
Restore the iPhone to the current firmware and then when you set it up restore the backup to it.

I've done this with my iPod Touch and it worked fine. Angry Birds and the like still had all the completed levels.
No reason why it shouldn't work for you but don't be mad if it doesn't