Unfortunately my Edge 840 is bricked - but I still can access it via MTP.
I would like to try whether a manual upgrade would work, but I don't have a suitable firmware file and Express and Webupdater both fail me.#
Put an empty file named "force.tmp" and the firmware file "GUPDATE.GSP" of the version you wish to revert to into the \Garmin folder on the device, then disconnect device. During startup the device prompts for installation/reverting the software version. All settings are lost, but this is not new.
** Note Edge devices cannot be downgraded to a software version from the “before MTP era” using a GUPDATE.GSC file. If copied to the \Garmin folder nothing happens after restart and the .GSC file is deleted.
No guarantee / own risk.