Quickest way... Pull plug wire off the spark plug, pull the rear bolt out of the left side (as you sit on the bike) chrome trim on the top of the engine. Gently lift up that corner of the cover and swing it outwards. Some bikes it will move, some it won't without loosening the front cover bolt. If it will swing, use the spark plug socket and correct wrench that came with the bike to remove the old plug. If it doesn't swing, loosen the front bolt, 10mm socket on short 1/4" driver on the bolt head, needle nose pliers to hold the lower nut.
Install new plug and reverse the above steps.
Factory method? Remove seat and gas tank. Pull wire off spark plug and change the plug. What they don't tell you is some bikes need to have the fuel drained and the petcock (gas valve) removed before the tank will come off...
If you haven't done this before have a couple band-aids handy... The backside of the chrome cover can be razor edged.