There should be an adjustment screw and jam nut that sit right on top of the valves in the rocker arm. Loosen up the jam nut slightly so you can turn the adjuster screw, when you have the screw properly adjusted, hold it still, while carefully re-tightening the jam nut. You will need a feeler guage and the valve clearance specs to do this properly. The specs may be in your owners manual, if not, you should probably get a repair manual anyway if you work on your own machine.
Yours truly, sincerelly, love always, Josh aka The DirtDemon.