Download the correct software from Ubiquiti.
Change the name from the downloaded file to fwupdate.bin
Open WinSCP and connect to the AP using SCP protocol.
Copy the fwupdate.bin file to /tmp folder.
Close WinSCP and open Putty. Connect to the AP using ssh.
Issue the command syswrapper.sh upgrade2 & after this your session will close automatic and the AP will start the upgrade.