Engine should be off, hold down both buttons at the end off stork.
Switch on still holding buttons down then switch off then press buttons ten times. The new mileage will show.
If it’s just the spanner flashing on the dashboard then it’s easy to reset.
Scroll through display using button on end of wiper stalk until you get to the remaining mileage.
Press and hold button until display flashes to new service mileage.
Keep button held until the spanner stops flashing and let go of button.
Turn ignition off and on to check, bingo!!
You can reset the SERV/MIL light by disconnecting battery negative for 20 seconds but you need to recode your radio, and if fault is still active it will come back on again