From the Garmin forum:
Originally Posted by JJJHAWK88 View Post
Just to be sure we have instructions on how to revert back to 3.10 software in once central location, the following is what you will need.
Please download software version 3.10 from the following link:
http://www8.garmin.com/software/Edge605_705_310.gcd * Save the file to your desktop
* Rename the file GUPDATE.GCD
* Plug your Edge 605/705 into your computer
* Drag and drop the .gcd file on your desktop into the Garmin directory on your Edge 605/705
* Unplug your Edge 605/705 from your computer
* Power your Edge 605/705 on
* Follow the on-screen prompt and then allow for the software to back date
This is one of the threads that may lead you to it:
https://forums.garmin.com/...t=firmware+downgrade The link may be dead. I may still have the 3.1 file lying around... otherwise look for it on the Garmin site.