There should be a screw to the side of the valve that can be loosened so that you can adjust the valves. some valves can be adjusted without the displacement of a screw.
Remove the valve cover, using a torque wrench and tighten rocker arm rods to the specified torque value. Replace the valve cover using a new valve cover gasket if needed.
Adjusting valves on overhead valve engines of any type involves using a feeler gauge of the proper size based on the specs for the engine you are working on.