How can I adjust the alarm sensitivity on a Jmstar 150 cc scooter

Follow your throttle cable to the carb. You'll see a screw that pushes a pulley down raising your rpms

Generic videos on Chinese scooters in the link.

https://www.google.com/search?q=Replace+battery+on+a+2009+150+cc+benzhou+scooter

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Try these guys. they're pretty knowledgeable.
http://scootdawg.proboards.com/board/2/250cc-scooters

Did you try to clear fuel filter if it is fit. As well it may be problem with fuel injection, either pump or difuser.

Operate the dimmer switch--does one beam come on? Check inside the headlight shell and examine the wiring from the lamp socket into the handlebar and main harness wiring. If individual connectors, check that the colors match at each connector. If you can't find any problem, then the wiring is mis-connected. Make sure the lamp socket is connected to ground.

at the control module under the hood usually there will be a know or a screw of sorts to adjust sensitivity

TAKE IT TO THE DEALER AND THEY HOOK IT TO A MACHINE TO TEST IT