Its well easy, take the clocks apart, undo the little cog from the side of the mileage meter and rewind back. I used a dremel with just the rubber tip fitted and clamped it all up in various vices and left it to spin away while I made I cup of tea. Going from 140 to 97 will take freaking ages this way tho, cos affectively you are turning the dials thru 50,000 miles haha!
I wouldnt wana dismantle the whole thing and change the numbers individually, that would get bvery messy and probs wouldnt work again.