Old firmware versions available here: [Only registered and activated users can see links. ]

Download the version you want, then rename the file to GUPDATE.GCD, and copy it to the Garmin folder on the Edge.
Then unplug it and switch it on, and it should install that firmware version.

Downgrading will overwrite your personal settings, so back them up first if you want to keep them.
See here [Only registered and activated users can see links. ]

If you do not know what you are doing you will brick it then you can be put in the bin.

Too many user's brick there Garmin units and want other's to get them out of their mess.