Check the detector type and battery compartment location
Turn off power to the alarm if it is hardwired
Twist or slide the detector off its mounting bracket
Open the battery compartment
Remove the old battery
Insert the new battery with the correct polarity
Close the battery compartment
Reattach the detector to the mounting bracket
Restore power if it was turned off
Press the test button to confirm it works
Replace the battery immediately if the low-battery chirp continues