Do you mean the Odometer? The little wheels with numbers that indicate how many miles are on the vehicle? It is a federal crime to reset that or tamper with that gauge in any way. No matter what you've done to a vehicle, even if you've torn it down to bare parts and rebuilt it, the amount of miles on it, is the amount of miles on it, period.
So far as in setting the speedometer to the right position, I did it in my mustang by taking the lens off the instrument panel, and hooking up my scanner to the car to see it's speed coming from the computer, then pulled the gauge needle off it's little pin, and placed it correctly according to my scanner, which has to be one that views live data.
