Can you tell us what type of account you are wanting to change?
Is it your iTunes?
If you open iTunes then click the iTunes Store tab and select the Sign in option and enter your username and password XXXXX can you click on the iTunes Store tab again and select the View my account option and Re-enter your password XXXXX prompted.
Once you are into the store view can you click the Edit Payment Information button on the main screen and then either delete your card or if you want to enter a different card can you select the credit card type you want to use and fill in the details then hit done.
If you need more info let us know