Remember that it is not possible to modify an invoice.
Option 1: Modify the footnote of only one invoice
Find the invoice you wish to modify and click on "Edit this invoice's footnote"
Option 2: Modify the footer of all invoices
On the Settings tab > Billing > PDF Invoice
Select the footer you wish to add to all invoices > Update
The bottom of the invoice will appear on future invoices, but not on invoices already generated.