1.
copy with scp/winscp flash-upgrade.bin 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
chmod 700 /tmp/flash-upgrade.bin
/tmp/flash-upgrade.bin

flash-upgrade.bin is a self flashing binary to override ubnts obscure security and flash protection mechanisms and signature checks to prevent flashing. 
ubnt may disable this method in future AirOS versions. we will provide new methods then