Replacing a PIR does not require a reset. Is the PIR not working? Or is the zone it's on showing as faulted / open at the keypad?
I wasn't able to locate an Aritech CS-250 Installation manual with a quick search, but since you didn't ask for a User Manual, I assume you have one. It will tell you how to arm your system so the interior devices (PIR's) are ignored while you are home.
As you're probably aware, the CS-250 is a very old control panel and it's been probably twenty years since I worked on one. I used to have an installation manual but I don't know if I still have it or not. In any case, it's doubtful if you actually need it. If there's a problem with the new PIR working, it's most likely either the wiring or the sensor. I can't remember off the top of my head whether the CS-250 used End-of-Line Resistors (EOLR's) but sometimes those can get misplaced/ wired wrong during a replacement.
Why did you replace the old PIR? "Because it was bad" is not a helpful answer. A helpful answer would be something like, "Because I had lots of false alarms on that zone", or "Because it wouldn't trip the alarm when I walked in front of it," or "Because I accidentally spray-painted it."
Why, specifically, do you think you need to reset your control panel? (Or were you thinking of resetting something else?) What is your system doing that it shouldn't do, or not doing that it should? And have you been using this system for years, or is it new to you?
