Our alarm has a message come up on it “ check low batt” Where is the batter?

assuming the system is wired the battery referred to will be the control panel backup battery in the main control panel, usually situated near to your electricity consumer unit (understairs cupboard or similar) again assuming the system has a separate keypad to operate it.
might be better to tell us the make/model
of system for better advice.

alan
May 2020