There are 2 possible scenarios as we have our billing on 2 different systems.
If you see the following "FastSpring account" link, head to Scenario #2.
Click on the "Lite/Standard etc. subscription details" link.
Then, click on the "Edit Subscription" button. If you can't find the link to click, hover your mouse near to the Edit Subscription words until a hand-click icon is shown:
Select "Update Payment Method" and click "Next".
Update your credit card details accordingly or input details of your PayPal account. Then click "Update Payment Method".
Click on the "FastSpring account" link:
Here you will see all details of your order:
Switch to Account Details and Payment Methods tab to see what payment option you currently have on file:
If you need to add a new card or tie a PayPal / Amazon account, you need to remove the current payment method and add a new one:
In case you have more than one payment method on file, you'll need to head back to the Subscriptions tab, and click on "Update payment method".
Click on the "Use This" button on the payment method that you wish to use for your active subscription.
The final step ensures that you have a payment method tied to your current active subscription.