1. On Settings > Billing click the Default card on file image or Manage cards to open Cards on File ("Click any card to make it your default payment method.").
2. Click the row of the card you want to use (the current default is highlighted in green with a Default badge).
3. Confirm "Change default card - Are you sure you want to set this as your default payment method?" with Set as default.
4. You see "Success! Card on file was changed." and the card image on the Billing page updates to the new card's brand, number and expiry.

Future subscription renewals, add-on purchases and credit top-ups are charged to the new default.