Since I had to search a bit myself I figured it was worthy of a post for others that run across this:

1. Login to your database through phpMyAdmin or Sequel Pro, or whatever you happen to use.
2. Then find the #_template_styles table where # = the random prefix for your database.
3. Rows with client_id = 1 are you admin template styles and client_id = 0 are your regular site templates.
4. Change the value of the home column from 1 to 0 for the current template, and 0 to 1 for the template you'd like to set to the new default.

Source: adminpraise.com