Few alarm problems?

Hi there, I have an optima plus alarm (early 90's version) and for as long as I've lived there its been stuck in attack I personally didn't get the alarm installed it was already here, now when the electric goes the outside old siren rings which for its age is really good + when the electric goes the panel is still lit up but you can see there is a major drop in current which clearly tells me the battery is coming to an end I am unable to reset the alarm because it is completely locked does not respond at all

can anyone advise me on how to get it going again also I have no idea what the code is so that would be a great help as well

thank you