This is one of those things where the procedure's easy, but I use it so infrequently that I always have to look it up.
When a service is due, the S60 will display the message "Time for Regular Service". This documentation details the procedure to reset it once you've done the service.
The procedure's fairly simple, but the timings are quite tight, so read over the steps before starting
- Sit in the car with all doors closed
- Set the ignition to position 1
- Make sure the trip is showing T1 with a non-zero value
- Press and hold the reset button for the trip (the left button)
- Mileage will appear to clear, keep holding the button in and turn the ignition to position II within 2 seconds
- Continue to hold until the mileage re-appears and the orange warning light starts flashing
- Release the trip reset within 4 seconds
- My car beeps once the value has been reset (though apparently not all do)