Update an existing payment.

Payment status lifecycle is as follows: PENDING: The initial status of an ACH payment put towards a Credit Key order. CANCELED: Cancels an order installment payment that's currently in PENDING status. RETURNED: This means the ACH payment was returned in some kind of failure state. i.e. NSF, Unauthorized Bank Account. PAID: The ACH payment completed successfully. REFUNDED: The funds for a PAID payment were sent back to the payer.