Answer:
You need to do it by replacing the tappets between the cams and the valve cups. Theoretically it can be done without removing the camshafts if you position the can to point towards you and press the valve cup with a screwdriver while you grab the folding tappet with long nose pliers. You need a set of tappets to adjust the actual clearance or you may try to swap them get the right one for each valve. You also need a feeler blade to check the clearance and have to do all of this on a cold engine (few hours of cooling of were running previously).