You could try to adjust the clutch free play. If that doesn't help, you have a problem with the shift linkage or the transmission, itself and you will have to tear it down to find the problem.
To adjust the clutch free play, find the adjusting screw on the right side of the engine. Loosen the locknut and slowly turn the adjusting screw counterclockwise until you feel resistance. Now, turn the adjusting screw clockwise 1/4 turn. Lastly, hold the adjusting screw while tightening the lock nut.
I really doubt the clutch free play is the problem, but I would try it anyway, before tearing into the engine.
Most 4-wheeler problems are caused by a loose nut connecting the handlebars and the seat!!
You only need two tools in life -- WD-40 and duct tape. If it doesn't move and should, use the WD-40. If it shouldn't move and does, use the duct tape.