It's youth slang for sorry I must have been wrong old chap, please accept my apologies and I'll take myself outside and give myself a jolly good thrashing.
Very well, but you'd save money straight away as a company would only change the batteries every 4 years.
The XPC is most likely a dummy external siren as these were sold as monitored systems, sold and installed in the main by subcontract sales companies in the early 2000's so having one non monitored is a bit of a waste of time.
Just imo of course.
Unfortunately, and for obvious reasons, we can't advise people how to do this, likewise it will need reprogramming for your requirements in any case.
If you found the code to this system then potentially a lot of other systems by this installer could be compromised, how long ago was it under contract?
I'd suggest a reputable company to come and programme it for you, they will need the original installer code or they can crash and reprogram it. Get it serviced at the same time if it's not been done for a while (12m is recommended) and this will ease the pain of the call out to a non maintained system.
May also require a new back up battery if its older than 4 years.