To update your credit card number, follow these steps:
1) Make sure you're using the latest version of iTunes. It can be downloaded free of charge from the Apple website:
Note: Installing the latest version of iTunes will not affect your library or any items in your account that you haven't downloaded.
2) Click this link: http://phobos.apple.com/accountSummary
iTunes will automatically launch and the sign-in dialog will appear.
3) If you are not already signed in, you will need to enter your iTunes Store account name and password and then click the Account Info button.
4) On your Apple Account Information page, click Edit Payment Information
Make the necessary changes. Your billing address should be exactly the same as the address on your credit card statement and with the United States Postal Service (click "Look Up a ZIP Code" at http://www.usps.com and follow the onscreen instructions). This is important, because even differences in punctuation, abbreviation, and capitalization can cause payment errors that might prevent you from using the iTunes Store.
5) When all of the information is up to date, click Done.
I hope this helps