To change your billing details:

1) Go to the Billing settings in your account

2) Click the "Access your billing information" link

3) In the "Payment Method" tab of the "Active Subscriptions" table, click the "Change" link

In this screen, you can add/remove a card and decide which card to use

Note: If you are paying via a credit card attached to your Paypal account, you need to change the billing details in your Paypal account.