HA ha, reading an old forum , take the battery out, then power down the system, re-power and within 60sec press * + # simultaneously. This puts it int programming mode, then press #20 and the digits displayed are the installers code, this is the highest level code. EXit programming by pressing *99.
When you change the master code you have to press the keys with no more than a 2 sec puase between digits. AHH the internet is a wonderful thing.