Open up the AppStore, tap on the Featured tab on the bottom, and scroll all the way down until you see AppleID: <insert your AppleID email here>, then tap "View Apple ID" and log into your account to access your Account Settings. Then tap on Payment Information and you should be able to edit there on the screen.
Alternatively, you can also go to Apple's AppleID page
here and change your information there if you don't want to do it directly on your device. Both methods will net you the same result, just that one can be done using your computer and/or iPad, while the other is on your iPad directly. You can also use iTunes to change your payment information as well.
