How? Just turn the knob. But are you asking where you get the value of QNH? If that is the case you can get it from the nearest airfield or London Information both of whom who will pass you the Regional QNH or the one closest to you. The next question is when. Personally, I'd do that on the ground and update it as either the pressure changes locally or I fly elsewhere. The benefit of doing so means your charts and altimeter match. Both your altimeter and your charts have the same datum.