I think it is in the service manual, but it goes something like this.  You
press the T1 and T2 mileage indicators to zero.  Then you turn the ignition
key to the first position, hold in the mileage indicator button while you
turn to the thrid position (Key), and it should reset the service indicator.
This is from memory which at my age is not reliable, but in any case, it
goes something like described.
Sorry if it does not work.