there are 4 bolts on the back of the swing arm, loosen them in the order that it says in the manual then stick a screwdriver through the rear sprocket into a hole in the swingarm. roll the quad foreward to loosen and back to tighten.
Take an air gun and blow all the dirt out of the socket heads, loosen the 4 bolts. Use a 10.mm dia. or equivelent shaft or long bolt that will pass thru the sproket. With the bike in neutral move it so the sproket contacts the long bolt and adjust the play, check the manual for the correct chain play and where to measure it. When adjusted follow the tighting sequence and torq spec. You'll want to re-check the torq on the bolts after riding a while. DO NOT OVER TIGHTEN! If you ride a lot of mx or rough stuff you'll want to stay to the loser side of the spec as prevent any bind when the swing arm fuly compresses.