Depends slightly on the age of your 9-5 and whether it has a petrol or diesel engine.
But you have to press and hold the "Clear" button for about 10 seconds when the message is displayed.
If the car is 1998 to 2005, the clear button is on the dash display. You need to press once to clear the message, then press and hold. It will beep once and say "Cleared", but keep holding the button until it says "Serviced".
From 2006, the Clear button is on the steering wheel. Same press and hold, I don't think that you need to clear the message first. Green dash display will say "Cleared" and return to the normal display, but ignore that and keep pressing. It will then say "Serviced".
Once you see "Serviced" you can let go and the service interval is reset.
If you have a diesel engined car, you need to have your foot on the brake pedal to do this.
If you ignore the message, it will go away on its own eventually. Something like 20 trips.
