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 CORE Help Center for details.
To change an invoice template at the project level, check CORE Help Center > Projects.
To change an invoice template at the global level, check CORE Help Center > Global Settings. Under Template Assignments, you can select the template you want to use for each invoice type. If you have a custom invoice template, you can assign it to the appropriate invoice types and click Save when done. Check CORE Help Center for details on setting default invoice templates.