how do I update my credit card information?

Easiest way is iTunes.

1. Open iTunes.
2. Choose Store > Sign In.
3. Enter your Apple ID and password, then click the Sign In button.
4. Choose Store > View My Account.
5. Enter your account password, and click the View Account button.

To update your name, billing address, or payment information, follow these instructions:

1. From the Apple Account Information page, click the Edit Payment Information button.
2. Edit the information that you would like to change.Note: The payment methods that the iTunes Store accepts can be found in the payment type section. If you do not want a payment method on your account, select None in the payment type section.

http://support.apple.com/kb/ht1918

Jesse