21 year old Veritas ?

Hi

21 years ago I had a veritas r8 installed in my converted barn house for the last 10 years I rented it out now I allowed the tennants to change the code if they were willing to call a engineer now I need to change the code again due to moving back in but im not too sure on how too this as I have no maual

also like a idiot I replaced the battery not realising it would go off if removed but it only sounded for 20 I also removed the lid on the bell box to clean it cos I couldnt see the red l.e.d as clear I also want to replace the battery in the sounder its a flashguard 101 sounder card is there a special type of battery for these