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 or My Default Campaign.
Choose the Template or Campaign you'd like to set as default.
Click Update Default Options. This would only update the template/campaign for all future leads.
If you want to update the default template/campaign for existing leads this article should help: How do I change the mail campaign or mail template for a lead?
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.