In the Admin UI, you can change the default payment profile for a subscription by visiting the Payment Details tab and clicking the “Make Active Payment Method” link.
In the API, however, it wasn’t possible to do this. If the default payment profile on a subscription was deleted, there was no way to designate a new one as the default.
As of July 13, 2016, you can use the
/subscriptions/<id>/payment_profiles/<id>/change_payment_profile endpoint to designate any of the Customer’s payment profiles as the default for a particular subscription.
For more information, please see Change Default Payment Profile in the API documentation.