Determine the battery type. In order to change the battery, you must be sure you have the right type of replacement battery. Most smoke detectors use 9-volt batteries, and carbon monoxide detectors tend to use four AA batteries. A security alarm might use a specialized battery that you must acquire from the manufacturer or the alarm maintenance company. In some cases, you might even have to let the alarm company do regular battery changes on your security system, in which case you should mark your calendar to remember to call them within the recommended time frame. If you don't know what type of battery your alarm takes, open the alarm to look inside. In some cases, a screwdriver might be necessary for this step.
Clean the inside of the alarm. This is particularly applicable for smoke alarms. In a smoke alarm, dust or dirt inside the alarm can float through the sensor, causing it to go off. A quick dusting to the interior of the alarm can help prevent this. If any corrosion has leaked into the inside of the alarm from the old batteries, clean it thoroughly. If corrosion is particularly pervasive, it might be time to buy a new alarm.
Place the battery according to instructions. Alarms come with instructional fliers that provide directions for changing the batteries. If you have any doubts about which direction the batteries should go in, consult the diagrams in the instructions. Most alarms will provide a chart on the battery compartment itself showing which direction the plus and minus ends of the batteries should lie. Install the batteries and replace the battery cover.
Test the alarm. Before declaring the task complete, check to be sure the alarm works. Most home alarms have a test button that, when depressed, will set off the alarm for a short time. You might want to muffle the alarm with a towel at this point so that the loud sound of the alarm is less disruptive. Once you've tested the alarm and are certain it works with the new batteries, your job is done.