Unfortunately there is not a way to lower the amount on the payment at this time, so you would need to create an entirely new payment if you wished to change the amount. Also, as for the CompletePaymentRequest not existing, this is a known issue in the API Explorer (reported late last week) and we’re working to remove it. In the actual SDK, no model/object is required, you simply pass the payment_id to the CompletePayment function.
I’ll DM you for more information on your use-case, though.