The current guide will show you how to set an invoice template as default.
Global setting
To configure it globally (for all projects), please go to Administration (on top) - Invoices - General. Here, on Invoice template set the needed one and save the changes.
Local (project) setting
You are allowed to make this configuration on a local, i.e. project level too. So, please go to your Project Settings - Invoices tab.
Note: Please keep in mind that project configuration overrides the global.