If you purchased access to a paid experience in a Mighty Network where you are a member and you need to update your billing info, you can do so in a few easy steps.
First, click on your profile picture and then "Your Purchases"
From this menu you'll be able to see all of the purchases you have active on the network. Click on an item in "Upcoming charges":
From there you will be able to see the details about any upcoming charge you may have. You'll also find a handy "Edit Billing Info" button under the Billing Information header for you to change your payment information.