Fix Unifi Cloudkey unable update

1. Connect to the UniFi Cloud Key via SSH.
2. Issue:

   dpkg -l |grep ubnt-tools

3. If ubnt-tools is anything other than 0.9.7-1, then you need to update it. You can update it by issuing: 

   apt-get update && apt-get install --only-upgrade ubnt-tools

4. Once the ubnt-tools package is updated, you can run the upgrade by issuing: 
   
   
   ubnt-systool fwupdate http://dl.ubnt.com/unifi/cloudkey/firmware/UCK/UCK.mtk7623.v0.8.9.d9dda97.180108.1229.bin

ref: https://community.ubnt.com/t5/UniFi-Routing-Switching/Can-t-update-Cloud-Key-Firmware/td-p/1781861

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1) Download the 0.8.7 firmware from here.
2) Save the file on your desktop, and rename it to fwupdate.bin
3) Open a SFTP connection to your cloudkey (Im using filezilla for this). See screenshots (File1/File2)
4) Browse to the /boot directory and drag the fwupdate.bin into the red area. See screenshots (File3/File4)
5) Reboot the cloudkey.
6) Now upgrade your UCK to the latest firmware available.

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Go to : etc/apt/sources.list.d

Edit source list > Add the line:

deb http://www.ubnt.com/downloads/unifi/debian cloudkey-oldstable ubiquiti