When you have first setup your iTunes account and tried to make purchased from the iTunes Store, you had to supply valid credit card information other form of payment method, and enter your full billing address details. You can of course change your credit card information from iTunes itself, but you are also able to update your payment information from an iPad, as explained in this tutorial.
Follow these steps to change credit card or payment information:
Tip: for your protection and security, your iPad (and the iTunes Store) will automatically timeout your session. In other words, if you stay with the Edit Billing Address dialog open for too long without doing anything, you will receive a "Your session has timed out. Please try this operation again from the beginning" error message; all you need to do is tap on "Done", start over from the top of this tutorial, and then edit your payment information as soon as the screen opens.
This is all it takes to update your credit card and other billing / payment information from the iPad!