Open your DealMachine mobile app or log in to Dealmachine.com from your computer.
Click the menu button in the top left (mobile only).
Click Default Mailing Options.
Click My Default Template.
Click Update Default Options.(You can either save it as default or save it as default and update existing deals as well)
Hope this helps!
If you need to create a new template, navigate to the menu button in the top left (mobile only). Click Templates. Click Create new template. Save. Then set the template as default as described above.