To replace the battery in your Simon 3, follow the steps below:
1. Disconnect the power from your system by unplugging the transformer from the nearby outlet.
2. Push down the tab on the top of the Simon 3.
3. Open the front casing of the system.
4. Take a flat blade screwdriver and place it under the tab at the top of the Simon 3, and pry the second casing open.
5. Remove both wires from the battery.
6. Remove the screws from the sides of the battery compartment.
7. Remove the top cover of the battery compartment.
8. Remove the battery.
If you are adding a replacement battery for your Simon 3, just follow the steps in this article in reverse.