GE Simon 3 Add or Change a Sensor to my system?

You need to use an installer code to add/remove sensors. Depending on what number of digits your alarm code is set for will determine the number for the intaller code. Look in the installer manual for the code. Then follow these steps:

Open the panel.
Key in installer coe
Select Add
Select Sensor/Remote
Activate the sensor you want to add. Some sensors have a button, some have to opened to activate.
Choose sensor name and group. Groups are listed in the manual. Basically, if you want the sensor to arm when the doors arm (such as for a window sensor or door sensor) put it in group 10 or 13. If it's a smoke sensor, group 26; motion detector, group 17 and keyfob, group 01.
Press done when finished and run the test to make sure the system is reading the sensor.

Your question is not specific. I think the manual is your biggest help for step by step instructions.