The first thing you need to do is log into your account page here

If you have an active subscription, then follow the instructions from the FAQ "How to change my payment method".

If you do not have an active subscription:
- Select the Subscriptions link from the top menu.
- Click on the specific subscription id you wish to change the payment method for.

- Click the Resubscribe button displayed right below the subscription details. This action will redirect you back to the checkout page where you will basically place a new order and you can fill in any kind of new payment method you wish.