It depends on what year the 156 is, on the early models it has to be done with diagnostic equipment.
It can be done on later models by pressing the INFO button on the dash until the service information is displayed, then press the minus (-) and plus (+) buttons together for nine seconds.
The service time and distance will be reset.
hi rojam thanks so much for your answer, i got the same answer as the one you have gave me from another site and the pressing the info button etc worked so thanks again