Check the smoke detector type and battery compartment
Turn off any power to hardwired units if needed
Remove the detector from its mounting bracket if required
Open the battery compartment
Remove the old battery
Insert the new battery with the correct polarity
Close the battery compartment
Reattach the detector if it was removed
Restore power if it was turned off
Press the test button to confirm it works
Replace the battery again if the low-battery chirp continues
Dispose of the old battery properly