| You can remove the side casing with the motor in the chassis, you will to drain the oill and you'll need a puller that goes on the end of the crank to pull the alternator magnet off the crank.
With the side case off you can check the gear selector linkage, this could have slipped or be worn, if you need the page from the manual to set it up U2U me. if it changes up ok and not down it is probably the selector.
It is a bit of a tricky job without the right tools but can be done if your mechanically minded and prepared to spend plenty of time on it.
Good luck. | 
wilf
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