Edit an invoice
Login and go to Sales > Invoices. Filter or otherwise find the one you'd like to edit.
Click on the invoice and you'll see a nice representation of the final invoice that is ready to be sent/was sent to your customer.
Click on the "..." dropdown at the top right of the invoice, and select Edit.
The next screen is where you can edit the basic data of the invoice, add notes and other useful pieces of information that you need, as well as tag the invoice.
Click Save changes at the bottom of the screen when you're done, or Cancel to revert any changes you've made so far.