Veritas 8C Battery Replacement?

Hello,

Can you tell me how to replace the battery in my Control Panel please. The panel is over 6 years old and when we had a pwer cut after 45 mins the external sounder started.

I want to know if I should turn the main power off first and then replace the battery? That seems to make more sense to me but I read somewhere that you should not need to power down first. If I disconnect the mains and then the battery, will the external sounder not go off?