At main menu click on Settings >> Invoice settings tab
Invoice settings
Default invoice template: The invoice template that is set for new invoices (you can change it per invoice) You can preview the look of the selected template
Invoice prefix: Invoice numbers may optionally be prefixed with a group of letters (P.E invoice number may be: INV101)
Starting invoice number: if there are no invoices recorded yet then this will be the first invoice number
Customize invoice text labels: if you would like to change the various texts that appear on the printed invoice, modify these values.
Predefined terms and conditions text: the default terms and conditions text that is set for new invoices (you can change it per invoice)
When you've finished editing the fields, click on "Save settings" toolbar button.