For that you need to keep pushing RESET switch, while the unit is power OFF.
After that, power ON the unit, without release the switch. Wait until the 4 leds lights into 2-way blinking (few seconds after RED led lights on). When done, you can release the switch, it takes around 12 seconds.
Now you are on TFTP mode, and you can downgrade the unit.
It was hard to me to figure out as in every place those instructions are pretty vague and incomplete.
Code:
c:\Ubiquiti>tftp -i 192.168.1.20 put XM.v5.3.6.14295.120904.1243.bin
Transfer successful: 6842256 bytes in 9 second(s), 760250 bytes/s