input the codes into scantronic 9450 wired alarm?

Hi, I am trying to assist my 93 year old neighbour with his faulty alarm system. After a recent power-cut, his alarm went off, when power was turned back on. It appeared to re-set itself, but recently when he went to leave his house, the internal sounder went off, and he could not silence it at all, except to turn off his mains power.

I have traced a fault with the internal battery, which appears to be not taking a charge. On test it was only showing 7.2 volts. I tried to charge it but no joy. I have aquired a new Yuasa Battery which I intend to fit today. My neighbour is a little unsure how he should actually enter his codes. I do not think that he is doing it properly. Can some-one please tell me the sequence of events that I should be going through to replace and re-set his alarm.
He cannot afford to call in a specialist, and I think this new battery might fix the fault.

Any help would be appreciated, Tony