I have marked an invoice as paid in error

Hello Lesley-Platinump, 

Welcome to the Community page, 

So if you have marked an invoice as paid in error if you go into that invoice and into the received payment, you will see at the option of the recieve payment page an option that says more, if you click that then click delete it will delete the payment you have record by mistake and open the invocie up again ready for when you do recieve the payment for it.