How to delete app data from an iphone?

A game (giant boulder of death) is broken pretty bad.a lot of the stuff dose not work and it glitches. I tried deleting it and removing it from game center but when i download it back all my upgrades and points are still there and its glitched. How do i erase all existence of the app?

Press and hold any app button until all the buttons start to wiggle and a little x appears in the upper corner of each one. Press the x of the game you want to delete. You'll get an option to delete or cancel, and you choose delete.

Beware, though, that if you've synced it with your iTunes library, it will be back. Delete it from there, too.