how can i dismantle my accenta/optima system to take to my new home?

hi guys. i am moving home and want to take my alarm system with me. my brother is going to re-install it when he gets back from his holiday, but he would like it 'ready to go'. i just tried to remove the cover to the siren, so i can un-mount it from the wall, but it went off when un-screwing the 2nd screw, but stopped when i turned the screw back a bit, then put the code in on panel as that was sounding because of Tamper.
Can anyone let me know the quickest way to dismantle the system without the siren sounding for too long. Thanx in advance for any help