You can update your credit card details from the My Account section of the IceTV website. After you login go to the My Account > Auto Renew Settings menu. Then where it says Confirm password: enter your IceTV password and click on the Enter Card Details button. Finally, enter your card details and ensure that all fields are filled in correctly before clicking on the Confirm Details button.
If you have any problems doing this please don't hesitate to contact support.