On your name in the XMB menu (in the PSN bit) press triangle and then account settings (or something similiar) and it will bring up a new menu and you have to press billing info and then you can view your current payment details and you just change it there.
go to account management.
click on the top of the two
edit billing information (or something like that)
delete credit card
go back to main screen of account management
then do what you did to get the first credit card on there