In Cards on File (Settings > Billing > Manage cards) each non-default card has a trash icon on the right. Click it and confirm "Remove card - Are you sure you want to remove this card from your account? This cannot be undone." with Remove; you then see "Success! Card deleted."

The default card has no trash icon: Sprwt always needs one valid card to charge your subscription, so you must first make another card the default (click it, then Set as default) and only then delete the old one.