cburger -
Welcome to the Community!
I just had to do this myself recently - keep in mind you can't edit an invoice that has not been paid, cancelled, or marked as paid. All you would need to do is go into the History, click on the details for the sent Invoice, and then click on "Edit." Optionally, you can go to the Request Money page, click on the number for the invoice, and then select Edit.
Thanks!
Adrian
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