If your device is not showing an available update or is in a "disconnected" state, you can force the upgrade via SSH. UniFi Release Schedule and LTS - Ubiquiti Community
Let me know your exact Ubiquiti model if you need help locating the right UPD file for version 1.255. Upgrade Firmware Version 1.255 Ubnt -UPD- Download
Addressing potential vulnerabilities to protect your local network from external threats. If your device is not showing an available
You can update your UBNT devices using several methods, ranging from automated controller updates to advanced manual SSH procedures. Upgrade Firmware Version 1.255 Ubnt -UPD- Download