All of your subscriptions use a single billing method. You can change your billing information by clicking your username in the top bar, click "Account Settings" in the dropdown menu, and then click the "Billing Information" link in the left navigation.
To remove your payment method, you will first need to set all campaign subscriptions to the free "Peasant" tier. We cannot remove a payment method if you have any active subscriptions. Once this is done, please email us at firstname.lastname@example.org and we will remove your payment method as soon as possible, generally within 48 hours. We will email you a confirmation that this has been done. We will be adding a manual payment method removal button in a future patch.