To change the battery in your First Alert 9120LBL Smoke Alarm, follow these steps: Remove the Alarm: Rotate the alarm counterclockwise to detach it from the mounting bracket on the ceiling or wall. Open the Battery Compartment: Locate the battery compartment on the back of the alarm. Press down on the catch on the top left to open it. Replace the Battery: Remove the old 9-volt battery and insert a new one, ensuring the positive (+) and negative (-) terminals are correctly aligned. Close the Compartment: Gently close the battery compartment. If it doesn’t close easily, check the battery orientation. Reattach the Alarm: Place the alarm back on the mounting bracket and rotate it clockwise until it locks into place. After replacing the battery, press the test button to ensure the alarm is working correctly.