When drafting an invoice, you have the option to change the invoice number and add a prefix and/or purchase order number. Simply head to your draft invoice in the 'Invoices' tab, or tap to 'Create invoice'. At the top of the screen, you'll notice 'Invoice number' - tap this to open up a new screen.
From here, you can type in a prefix, edit the pre-populated invoice number and add a purchase order number. Once you're happy with what you've entered, tap 'Save' in the top right corner to be taken back to complete creating your invoice.
To help keep you organised, you won't be able to repeat invoice numbers. This includes attempting to repeat invoice numbers of deleted invoices, draft invoices and sent invoices.