• Upgrading firmware – https://help.ubnt.com/hc/en-us/articles/204910064-UniFi-Upgrading-firmware-image-via-SSH
• What do the LED Color Patterns Represents – https://help.ubnt.com/hc/en-us/articles/204910134-UniFi-What-do-the-LED-Color-Patterns-Represent-for-UniFi-Devices-
• TFTP soft recovery for bricked access point – https://help.ubnt.com/hc/en-us/articles/204910124-UniFi-TFTP-soft-recovery-for-bricked-access-point

Over SSH:

mca-cli
set-inform http://<ip-of-controller>:8080/inform

adopt it in controller and then run set-inform command again on device.

Using SSH:

syswrapper.sh restore-default

Device will reboot automatically and start with factory defaults (login ubnt, password ubnt).

https://help.ubnt.com/hc/en-us/articles/205143490-UniFi-How-to-reset-the-UniFi-Access-Point-to-factory-defaults