If the CK is on any firmware between 0.8.1 to 0.8.4 you need to update the *ubnt-tools* to 0.9.7-1 prior to attempting upgrade of the CK's firmware to 0.8.7.

1) SSH into the CK.

2) First check current ubnt-tools version
    'dpkg -l |grep ubnt-tools'

this will show the current version most likely 0.9.5 or earlier

3) To upgrade the *ubnt-tools* to 0.9.7-1

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

4) Then update to the latest firmware(current at Jan18)

    'ubnt-systool fwupdate https://dl.ubnt.com/unifi/cloudkey/firmware/UCK/UCK.mtk7623.v0.8.7.4158d32.171130.1346.bin'

this will upgrade the firmware to 0.8.7 and controller to 5.6.26(bundled with firmware)

5) If the update fails due to lack of free space issue the following commands:

    `apt-get update ; apt-get install aufs-tools'
    `/sbin/auplink / flush`

And then go back to step 4)