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
NOTE: If this does not work, try it without the s in https:
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