You can save an invoice as a Draft, preview it, and make as many edits as you need.
You can also edit a Finalised invoice. Click on the invoice to open the side panel and select Edit invoice. This will move the invoice back to Draft status, so that you can edit it, then finalise it again.
If you need to edit an invoice that has already been sent, click the invoice to open the side panel, select Unmark as sent to move that invoice back to Finalised, then follow the steps above to edit it.