If you need to edit an invoice that you’ve already sent, you will want to cancel the original invoice to avoid your customer paying an out-of-date invoice. To do this, click the blue drop-down arrow on the far right of the invoice and select "Cancel" from the drop-down. 

You can then, select Archived from the Type drop-down on the Invoices page and you’ll see the invoice you cancelled listed there. Click the drop-down arrow on the far right of the invoice and select "Duplicate". This will duplicate the original invoice, so you can use that as a starting point to make your edits. After making your edits to the new invoice, send the new invoice to your client. This ensures that each edit has an incremented invoice number.