Please note: this comes with zero guarantees and could potentially brick your device. Garmin does not recommend or approve this. The downgrade has been able solve my GPS issues on 27.09 (wavy paths, path being off by > 20 - 100m, slow (>1 min) to get location lock, loss of lock all-together in open areas)
Older firmwares
Shared 18.21 17.21 11.12 for 255 non music (not sure if these will work for 255 music model)
drive.google.com/.../1-AnnqFzK_5ZfEhq-OFubFSpKlkXC79vz
FR 255 music:
20.32
21.19
21.20
Steps (taken from this post, force.tmp is just an empty file with that filename, I successfully downgraded my 255 Music to 21.20):
0. Have a Backup of the your settings before continuing check the actual fw version + GPS CIQ BMX WHR ANT/BLE Garmin Hub version
1. Download version for your model from the link and unzip the files
2. Connect your watch to the computer, open the explorer and copy both the gupdate.gsp and force.tmp files to the GARMIN folder of your watch.
3. Disconnect the watch from the computer. After some seconds it will detect the files and propose to perform the "update"
4. will take several minutes on "Loading FIT files" + restarts
5. Disable automatic updates on the watch. Important: Also disable this on Garmin Express in the settings (update device: never !)
6. Re-pair the watch with the Garmin connect App
If new firmware supports this: Backup Up & Restore -> Device Backups -> select the backup and click "Restore from Backup"7. Resync: this will bring back a lot off settings, workouts etc …
8. All done
