To reprogram your car key, set the car on "program" mode, press a button on the key fob to synchronize the fob with the car, and then test the key fob to complete this process. This procedure may, however, vary from one make and model of the car to another.
To avoid any complications associated with reprogramming the key from outside the car, sit in the driver's seat while reprogramming the key. Insert the ignition key into the car's ignition switch, and turn on the ignition to set the car on the "program" mode while taking care to avoid starting the engine. If necessary, repeat this process until the car assumes the program mode. Most cars flash their lights when they are in "program" mode.
Once the program mode is set, press any key fob button to synchronize the car with the key fob. Turn off the ignition switch, and remove the ignition key from the switch. Test the operation of the key fob to see if it locks and unlocks the car locks. If the key fob fails to remotely operate the car, check it for any defects, fix it as needed, and then repeat the process to achieve the synchronization.