I don't see why not. If you alter the reading and then try to sell the car that would be fraud, but the mere act of examining your own property shouldn't be a crime. (I'm officially undecided as to whether this reasoning extends to the DirecTV case).
Absolutely, you can do whatever you like to it. However, you MUST disclose that the mileage indicated is not actual when you sell it. Otherwise, that would be fraud. On your title transfer paperwork, there is even a checkbox to certify or disavow the indicated mileage.