SEE: https://help.ui.com/hc/en-us/articles/204910064-UniFi-Upgrade-the-Firmware-of-a-UniFi-Device

If updating a Cloud Key (UCK Gen2, UCK Gen2 Plus) use:

ubnt-systool fwupdate <download link>

ubnt-systool fwupdate https://dl.ui.com/unifi/cloudkey/firmware/UCK/UCK.mtk7623.v1.1.13.818cc5f.200430.0950.bin

ubnt-systool fwupdate http://dl.ui.com/unifi/cloudkey/firmware/UCK/UCK.mtk7623.v1.1.13.818cc5f.200430.0950.bin

dpkg -l |grep ubnt-tools
apt-get update && apt-get install --only-upgrade ubnt-tools

or

wget https://dl.ubnt.com/unifi/5.7.28-5c442c6b54/unifi_sysvinit_all.deb
dpkg -i unifi_sysvinit_all.deb
rm unifi_sysvinit_all.deb

http://fw-update.ubnt.com/

https://help.ui.com/hc/en-us/articles/204910064-UniFi-Upgrade-the-Firmware-of-a-UniFi-Device