If it's the 420 Rancher, the oil filter is located on the right side of the engine, toward the front under a small cover. The cover has three bolts holding it on. Be sure that you watch how the old filter comes out and don't put the filter in backwards or it will block oil flow to the top of the engine and damage 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.