Download WinSCP if you are on windows. Load it and launch it. Use it to connect to your edge router and copy the firmware over to the unit, much like you could copy files in windows explorer or ftp. I would put it in /home/username if you can.
Once you have it copied over you can add the system image from ssh. It's been a while so I have forgotten what I ended up doing to get it going exactly but it should be basically this command:
add system image /home/username/firmware.zip
