Turn off the power to the fire alarm by flipping the breaker off if it is hardwired into your electrical system. Disregard this step if the fire alarm is not tied into the electrical system.
Locate the battery door on the fire alarm. Depending on the brand and model of your smoke detector, the battery door will be on the back or side of the alarm.
Slide the battery door open and remove the drained battery. Install the new battery in the same location you removed the drained battery. Most fire alarms use a 9-volt battery. Refer to the instruction manual for your specific firm alarm for more information.
Slide the battery door closed. Turn the power back on to the fire alarm if you previously turned it off. Push the fire alarm "On" button and press the test button to ensure the smoke alarm works properly.