ssh ubnt@192.168.1.15

returns

The authenticity of host '192.168.1.15 (192.168.1.15)' can't be established.
ECDSA key fingerprint is SHA256:...
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '192.168.1.1' (ECDSA) to the list of known hosts.
ubnt@192.168.1.1's password:

Determine the firmware to download from https://www.ui.com/download/unifi/.

Confirm it is a .bin firmware file.

cd /tmp
wget https://dl.ui.com/unifi/firmware/U7PG2/4.0.80.10875/BZ.qca956x.v4.0.80.10875.200111.2335.bin

NOTE: If you do not have internet access on the device, try the following:

1. Download the firmware on another machine.
2. Copy the firmware to the Ubiquiti device.

   scp /foldername/firmwarefile.bin ubnt@IP-of-UAP-or-Switch:/tmp/fwupdate.bin

   • For example

     scp BZ.qca956x.v4.0.80.10875.200111.2335.bin ubnt@192.168.1.15:/tmp/fwupdate.bin

cp BZ.qca956x.v4.0.80.10875.200111.2335.bin fwupdate.bin

upgradesyswrapper.sh upgrade2 & 

syswrapper.sh restore-default

/var/logs/messages

SSH intro

https://help.ui.com/hc/en-us/articles/204910064-UniFi-Upgrading-firmware-image-via-SSH

https://help.ui.com/hc/en-us/articles/204910124-UniFi-TFTP-Recovery-for-Bricked-Access-Points