How do I turn it off? Purchased a 2000 Grand Prix used and the owners manual was missing.

try pulling the negative terminal off the battery, then press the brakes a few times, and reconnect it. It should be gone when you start it up again

Here is how to reset your Oil Change Reminder Light:
1. Turn the ignition key to the "On" position.
2. Push the accelerator pedal to the floor & release 3 times in 5 seconds.
The "Change Oil Soon" light should flash indicating the system is resetting.
3. After the light has finished flashing, turn the igniton off then start the vehicle.
If the oil change light comes back on the monitor has not reset, repeat steps 1-3.