Setting a new default email template means the new email will replace the system default email that is automatically sent out.
Personal templates cannot be set as default emails. However, it is possible to create a copy of a default email, modify the template, and then set the copy as the default email. To do so:
Navigate to the Communication section and then the Modify Templates tab
Search for the original email that you want to create a copy of, select the action cog, and then click Create a copy
Modify the copied email template
Tick on Make this the default template
Click on Save Changes