UniFi Controller 5.10.12. SeeHERE for controller release notes.
How to update the latest firmware:
From UniFi Network Controller: Settings -> Maintenance -> CHECK FOR UPDATES (in CloudKey Firmware section) -> APPLY UPDATE
From UniFi Cloud Key WebUI: Manage Cloud Key > Settings > Firmware > Update Manually. From here you can either paste the URL, or if you downloaded the binary you can select the file locally on your computer.