Default Email Builds will appear at the top of the list when drafting messages or will be the email build used for transactional emails. A default email build can be selected for each mailing category. As you continue to create emails within Email Builder, you can choose which email builds appear as this default.
Go to Communications > Email Builder.
Search for the email build you would like to set as your default.
Click on the Make Default button to the right of that email build.