You can choose a default invoice template in Global Settings or assign a different invoice template to a project at the project level. You can also change the invoice template at the last minute from the Invoices screen. However, this change is not permanent and you can use it to temporarily view or email the invoice in a different format. Check the Core Help for details.
To change an invoice template at the project level, check the Core Help.
To change an invoice template at the global level, check the Core Help.