replacing new batttery in 9450 alarm system ?

how do i replace a battery pack in an 9540'thats not worked for a few years i am a new owner of the property so wont know the old owners code, old code so how can i overide the old code and how do I disconnect the battery safely? downloaded onstructions to this alarm have followed all the engineers codes factory default nothings Happened, no sound, no flashing of lights opened up the panel with a sceprewdrivee found instructions behind the wires cant reach them battery looks dead anyone help please?