I hope you are handy or have a friend who can help you... You have to take off the complete rear assembly... screws on bottom of chassis, comes out as a unit... the rest is easy, make sure you have a snap ring plier.
The best way to mesh the gears is to take a piece of notebook paper and fold it in half... when you are ready, place this between the gears and the spur gear... push tight and tighten bolts on engine. remove paper... now you should have the proper mesh.... once you get good at this, You should be able to change gears in about 10 minutes and adjust gear mesh in about a minute or two......