Scantronic 9800 - How do I replace a PIR?

One of my PIRs is not working after it has been moved because of some building work and I need to replace it. I think I can cope with the wiring but I need some advice on how to disconnect the old one and reconnect the new one without the alarm going haywire. Is there a process that I need to follow when working on the system to put it into some sort of maintenance mode to prevent the alarm triggering? I would be grateful for any advice and help.
Patrick