This article explains how you can add or edit your billing information
When logging into your Spark Site, select Marketplace and then select 'Update Billing'
Once you're in your myaccount area, select 'Add New Credit Card'
Enter your new credit card details then select 'Add a new address'
After filling out the billing address modal, select 'Save Changes' and when back in the Payment Method window, select 'Save Changes' and you're done!