Change Your Plan's Payment Method
While Changing your Plan, you will have the option to select your plan's payment method.
Select an Existing Payment Method
If your payment method has already been added (and verified in the case of the bank account method, expand the dropdown at the bottom of the Select or Change your Plan modal window.

Once the desired payment method is selected, Press Update.
New Payment Method
If the payment method you want to use has not already been added, you can Add a New Payment Method.
Alternatively, you can use the + New Payment Method button in the Select or Change your Plan modal window.
