once it is serviced the light should be switched off by the computer being plugged into your ecd (cars computor) but it is only your ford dealer who really has these machines.
Sorry DONNA1458.
Easy one to turn off, you don't need to take it to Ford's to plug it in.
To the left of the steering wheel just below the air vents, you'll see a round thing (for want of a better word). It has four buttons on it, marked
SELECT (the big one on the left)
CLOCK
RESET
UNITS
Switch the ignition ON.
Press and hold the Select and Units buttons together for approx 5 seconds. The warning lamp will illuminate for 4 seconds, the service light is now reset.