In order to delete an invoice, you first need to open up the profile of the individual associated with the invoice in question. To delete an invoice, go to People -> Search for the person's profile -> Finances tab on their profile -> Revenue.  This should show you a list of any invoices attributed to this person.  You can delete an invoice by clicking on the delete button (the trashcan icon) listed to the right of the invoice details. You can learn more about invoices here: http://nationbuilder.com/how_to_use_invoices