How can I change my bank details

On your computer's iTunes you should be able to edit your payment info by going into the Store > View Account menu option and logging into your account, and on your account's details page there should be a payment link. If you are doing it on your iPad then tap on your id in Settings > iTunes & App Store (Settings > Store on iOS 5) and tap on 'View Apple ID' on the popup and log into your account - that should also give you a payments link on your account's page.

Changing payment info : Change or remove your Apple ID payment information in the iTunes Store - Apple Support