How do I modify an invoice or quote that’s been finalized?

A document in a status other than “Draft” cannot be modified.

However, you can change the status back to “Draft” by clicking the current status, then selecting “Draft”. Once this is done, you will be able to once again edit the document.

Note that companies subject to France’s VAT fraud prevention law cannot modify an invoice once it’s finalized.
In that case, you can cancel the invoice then create a new one by duplicating the old invoice using the contextual menu located to the right on the page listing all invoices.