I want to change the debit card i use for app purchases. how do i do that

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 you could try tapping on your id in Settings > Store (Settings > iTunes & App Store on iOS 6+) and selecting 'View Apple ID' on the popup - that should also give you a payments link on your account's page.

Changing payment info : http://support.apple.com/kb/HT1918

Debit cards are no longer accepted in all countries, so it's possible that you will get a declined message, and will need to use a different payment method.