1. Find the payment. Go to Sales > All Sales and filter the type to Payment, or open it directly from the customer's own transaction list.
2. Decide what you actually need: void, delete, or unapply. Voiding zeroes the amount but keeps the transaction on record, marked voided. Deleting removes it from QuickBooks entirely, with no trace. Unapplying leaves the payment and the money alone but detaches it from the invoice it's currently applied to.
3. To void a payment, open it, select More, then Void. Confirm when asked. This reopens whatever invoice the payment was applied to as unpaid again, while leaving a visible record that something was recorded and then reversed.
4. To delete a payment instead, open it, select More, then Delete. Confirm. This also reopens the related invoice as unpaid, but leaves no record the payment ever existed, which matters if you've already reported on it.
5. If the payment itself is fine but it's applied to the wrong invoice, unapply it rather than voiding or deleting. Open the payment, uncheck the invoice it's currently matched to, and save. The amount becomes an unapplied credit you can then apply to the correct invoice.
6. Check for a deposit link before doing any of this. If the payment screen shows it's already part of a bank deposit, voiding or deleting it directly will throw that deposit's total out of balance. That's a different, more specific situation (see the FAQ below) and it needs to be handled from the deposit side first.