Hi jn001 

This can be a little tricky but let me walk you through it. 

1. Click Sales
2. Click Invoices
3. Click on the invoice you want to reverse the payment of
4. Click on the "1 payment" link under the paid stamp (if you applied more than one payment it will say 2 or 3 etc)
   
5. Click on the Date you want to reverse a payment on
6. Click More
7. Click Delete
8. Click Yes

Let me know if I can assist further!