Edit invoice

Hello all,

Customer has ordered 30 tables. The 30 tables were delivered. But a wrong invoice was issued. There were 60 tables charged on the invoice. In the sales order, for all 30 items in the "Schedule lines" tab, under Confirmed quantity, it indicates "2" instead of 1. That is why 60 tables were charged.

How do I go about changing/canceling the invoice to get the correct amount on the invoice?

Thanks for your help :)