I have an alarm system made by Napco, it is a GEM-P800?

How do I reset the main code for a Napco GEM-P800 alarm system. We inadvertently lost the four didgit main code and cant reprogram the system. I have tried the dealer code but it doesnt work. Any Suggestions. We are in New York, system was installed by Slomins but contract is over and they refuse to give any information.