Use Customer Payment Modify to modify customer payments.

When a customer payment is in the Initial status, you can modify all aspects of the payment, including the customer and the amount. When the payment status changes to a status other than Initial, you can only modify the status and the description. However, you can reset the payment status to Initial, provided that the payment has not been paid, bounced, or voided.

When a payment is set to the Paid, Bounced, or Void status, you can no longer modify the payment. At this point, the only way to correct the payment is to manually re-open all customer invoices linked to the payment using Open Item Adjustment Create, and then create a new payment with the correct details.