I was looking for how to revert and downgrade to v 3.00, and I found some instructions here.
Since this is hosted on a Garmin forum I figured I would preserve the information on how to downgrade, and also the firmwares here.
0. Download the firmware you want. Don't forget, this is for the Garmin Edge 500, not any other model!
1. Plug the Edge 500 into the PC.
2. Once the Edge 500 is ready, copy the file to the Garmin subdirectory on your unit, renaming it to gupdate.gcd. The 500 will appear as another drive on your desktop (Mac), file manager eg Nautilus (Linux), Explorer drive letter (Windows)
3. Save the following into your Mac / PC / Linux computer so you don't have to recreate stuff:
- Garmin\Sports\Cycling.fit (optional)
Updating the firmware doesn't usually mess with these files (a 'full reset' normally proscribed by Garmin support does).
4. Safely remove the Edge 500 from the Mac / PC / Linux.
5. Power up the Edge 500. If you are loading an older level firmware, it will ask you to verify you want to do this.
6. Let it update. This may take a while, be patient. Be very patient. Very.
7. Once the update completes, power the Edge 500 back down.
8. Reattach to your computer and restore the files saved in step 3.
9. Safely remove the 500 from the PC.
10. Power up the Edge 500, rescan for HRM, GSC-10, other devices, and re-pair them.