1.
It will generate a tamper (unless the battery is flat, flat, flat or you have the panel in installer mode). Enter your code to shut it up. Older PIRs use two AA, MN1500 batteries whilst the new ones use a single AA-sized 3.6V lithium battery LS14500. Scantronic recommend that you take the old batteries out and wait at least 5 minutes before putting the new ones in.
Replace the batteries and put it all back, then reset the panel.