Switch primary and backup credit card
You will need to go to your Billing tab, ensure you are under the Billing summary section and then click the Settings tab within this page. You will then be able to manage your cards and form of payment by clicking Edit next to Your selections.
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For that, first you need to go to theBilling tab, then click on Settings Tab (adjacent to Transaction History) in the front screen. Underneath, you will see Your Selections. Click on Edit and interchange the Credit Cards for Primay and Back Up. This should help you in choosing the right Credit Card as your Primary one.
Hope that helps!