Updating GPS

Well I tried to update the firmware on my garmin 18 lvc gps. This is the hockey puck looking gps that I plan on sending on the balloon. It needs a firmware update so that it can read altitudes above 60k feet. It comes with bare wires which I attemped to hook up via a breadboard to a 5v power source and a serial connector to a computer. I tried several times but could not get the firmware program to see the gps. I don't know if it was my shoddy wiring, the gps, or the computer. Arrrrrgh. I'll try again later...