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.
*This release follows our usual release structure which means it will initially be available via this blog post only. It will be posted to the download site and official repos in the near future. If you aren't familiar with our release structure, please take a moment to read our post HERE. Thanks!