To change the battery in your Gentex S1209F Hardwired Interconnected Photoelectric Smoke Alarm, follow these steps: Turn Off Power: Ensure the power to the smoke alarm is turned off at the circuit breaker to avoid any electrical hazards. Remove the Alarm: Gently twist or pull the smoke alarm from its mounting bracket to access the battery compartment. Open Battery Compartment: Locate the battery compartment on the back of the smoke alarm. Open it by sliding or lifting the cover. Replace the Battery: Remove the old 9V battery and replace it with a new one. Ensure the battery is inserted correctly, matching the positive and negative terminals. Close Battery Compartment: Securely close the battery compartment cover. Reattach the Alarm: Place the smoke alarm back onto its mounting bracket and twist or push it into place until it is secure. Restore Power: Turn the power back on at the circuit breaker. Test the Alarm: Press the test button on the smoke alarm to ensure it is functioning correctly.