How do I put a payment on the right customer after it was recorded?

Open the payment on Money in and choose the customer under Who paid. A cash sale you logged without a customer, or a bank deposit nobody was attached to, then counts on that customer's lifetime spend and payment count straight away, and choosing a different customer moves it off the first one. Only the account owner can do it, and Des can do it for you with an undo for five minutes. A payment that settles an invoice follows the invoice, so change the customer on the invoice instead, and a payment in a closed month stays as it is until that month is reopened.