IF you search e46fanatics, you'll find how to do it.
Since I have it handy, I'll post it here:
Press and hold the left button on the cluster button.
Then turn your key on to the first click.
This is know as KL15 position at this point they mileage on the service should be displayed in the instrument cluster.
Keep hold the button until the mileage begins to flash then you must release the button and press it again and this will reset the mileage on the service indecator.
BMW 645i - Black on Red
Range Rover Sport - Black on Black