Hardwired smoke detectors use a backup battery in case of a power outage. Replacing a hardwired smoke detector requires removing the battery. When you disconnect the power to the circuit breaker, the battery power takes over. The detector begins to chirp to let your know the power is not on. Eventually the alarm will sound. It is a good idea to replace the smoke detector with the same brand. This eliminates the need to attach a new mounting ring.
Turn off the power at the main circuit breaker in your home electrical panel. Place a stepladder under the smoke detector so you can reach the detector to remove it from the ceiling.
Rotate the smoke detector in the direction of the arrow on the detector. This will disengage the smoke detector from the ceiling mount. Untwist the wire caps connecting the smoke detector wires to the house wires. Place the caps back onto the house wires to keep them handy. Find the battery cover on the smoke detector, and remove or open the battery cover. Remove the battery from the smoke detector. Do not reuse the battery in the new detector.
Remove the screws securing the mounting plate to the ceiling junction box with a Phillips-head screwdriver, if using a different brand of smoke detector. Install the new smoke detector mounting ring to the junction box, securing the ring with the mounting screws.
Find the quick connect wires in your new kit. The quick connect will have two or three short wires attaching to a small connector. Secure the wires to the house wires using the plastic wire caps. Match like color wires together when attaching the wires.
Open the battery cover on your new smoke detector. Install the battery in the smoke detector and close the battery cover. Most smoke detectors take a 9-volt battery. Connect the quick connect connector to the plug in the top of the smoke detector. Position the tabs of the smoke detector into the ring and rotate to lock the detector into the ring.