If the invoice status is Completed, to add any new changes to it, you can go to Finance-> Invoices, open the desired invoice page. Then, click on More arrow and select Change to Draft.
Not that when you add the new changes to the booking, come back to the invoice page, and click on More arrow and select Re-generate Invoice.
If you have already Posted the invoice to Xero, to add new changes, you can go to Finance-> Invoices, open the desired invoice page. Then, click on Void from Xero.
