Change an invoice's due date or amount

Open the invoice and click "Edit invoice". Change the due date and/or the amount, add a reason, and save with the delete code. Paid and void invoices can't be edited. A part-paid invoice can be changed, but not below what has already been paid — if that payment was entered wrong, remove it first. If the invoice is a payment-plan installment, the plan shows the new date and amount too. Every change is recorded in the audit log.