Download the firmware from the website 2.4.0.bin
Download WinSCP and put the firmware on the /tmp/directory
run md5sum for the firmware to be sure it will match the hash from the download page.
if the md5 hash matches execute sysupgrade -n -v /tmp/2.4.0.bin, somewhere on the top bar will be a CMD box
wait for a reboot and after that you are ready.
Hope this help.