ALERT: This is NOT supported, so at own risk!
You cannot downgrade controllers unless you have a backup from the controller version you want to downgrade to (or older).
Always make a backup (suggest - settings only) to your PC before upgrading or downgrading so you can get back where you were.
NOTE: If you do not have a backup - then you will need to factory reset your devices, adopt and configure like a new setup.
Make a controller backup of settings only from the Backup tab and save it to your PC - so if anything goes wrong - just factory reset and restore.
SSH into the Cloud Key's IP and load the previous controller.
ssh ubnt@192.168.1.10 ubnt@192.168.1.10's password:
apt purge unifi -y rm unifi_sysvinit_all.deb* wget https://dl.ui.com/unifi/5.14.23/unifi_sysvinit_all.deb dpkg -i unifi_sysvinit_all.deb rm unifi_sysvinit_all.deb
NOTE: Replace the version in the wget line with the version you want.
NOTE: If you get an error when you try to restore the backup:
The restore is not functioning because you're uploading a newer controller backup on an older controller, that is not supported.
To resolve this issue, update the firmware to a version higher or equal to the version of your backup.
If you have issues, then check the logs:
/usr/lib/unifi/logs/ tail /var/log/unifi/server.log