Note: Each account must have a default payment method set and only one payment method can be set as default on an account. When billing is executed via a bill run or manually run (one-off billing) the default payment method will be used if charges are applicable.
On the left side in the Payment Methods panel click on the Payment Method in the list that you wish to set as the default payment method
On the right side in the Edit Payment Method panel enable the Default Payment Method option
Click the Save button