You can edit the Staff Profile and enter it here:
Past appointments won’t be updated as the invoices have already been created:
You would either need to add custom text to the invoice by clicking on the blue words that say “patient invoice” from the schedule or the billing area
and adding the number in the Details area:
That will then show on the receipt when you re-print it.
Or you need to unarrive and re-arrive the appointment to re-generate the invoice.