1.
copy with scp/winscp flash-upgrade to 192.168.1.20:/tmp
2.
login with ssh/putty to 192.168.1.20 and change directory to /tmp (cd /tmp)
3.
execute the following command
mtd -f write flash-upgrade.bin kernel0 && mtd -f write flash-upgrade.bin kernel1

once you need to recover, you need to reinstall the original ubiquiti firmware using the reset button recovery method.
then just start again. there is no direct installation way using tftp recovery since ubiquiti implemented a encrypted signature check (RSA)