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 phone then tap on your id in Settings > iTunes & App Store and tap '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 : http://support.apple.com/kb/HT1918