Every home should have at least two smoke alarms, and more if possible, depending on the size of the house and the number of rooms. A Firex smoke alarm is a very common alarm that can be battery-operated or hardwired. If you own the battery-operated one, you will need to know how to change the battery at least once a year.
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Remove the smoke detector by turning it counterclockwise to remove it from the mounting bracket. Grip the whole top of the smoke detector with your hand and turn it about a half inch. This will release it and you can pull it straight off. This will reveal the chamber where the battery is housed.
Pull out the old battery. You will not be able to mount the unit until you have put in a new battery. This is a safety feature that the manufacturer did to make sure you didn't forget to replace the battery. Open the little door that covers the battery. Typically, if you pry one end of the battery up a little with your fingernail, you can grasp it with your fingers to pull it out.
Put in the new battery, making sure the polarized ends are facing in the right direction. You can know this by the + and - markings on both the battery and the smoke detector. The 9-volt battery will have two prongs that need to be pushed into place tightly, while the AA types will face the same direction. Close the little covering door.
Place the unit back on the mounting bracket by setting it on straight and turn it clockwise until it sets into the groove, and then turn it clockwise about half an inch or until it stops and you can no longer pull the cover off.
Test the smoke alarm by pushing the large center test button. Hold it down firmly for about five seconds, making sure your ear is not too close to the smoke alarm. It should sound off for about 10 seconds. The little LED light should also be blinking about once a minute to show normal battery function.