Go to the Simon control panel and press the "Disarm" button underneath the "Home Security" mark.
Enter your code using the 5 buttons on the side of the control panel. Each of the 5 buttons represents two numbers: the top one is 1 and 2, the second one is 3 and 4, third 5 and 6, fourth 7 and 8 and fifth 9 and 10. To enter code 1234 you would press the top button twice and the second button twice.
Wait a second as the alarm system is disarmed and resets. The system is now idle and can be armed again.