Credit card payments
1) Go to the Billing settings in your account
2) Click the "Update payment method" link
3) In the "Active Subscriptions" table, click the "Change" link in the "Payment Method" tab
4) In the credit card information screen, you can add/remove a credit card and decide which card to use if there is more than one.
Paypal payments
Log in to your PayPal account and update the payment method in the PayPal interface.