Start by loosening the set screw marked 1 below. Depending on your brake, it should use either a 1.5 mm or 2 mm allen wrench. You do not need to remove it entirely, but do loosen it quite a bit.
The adjustment on the sensor should be such that the brake lever disengages the motor almost as soon as you pull the lever. To make the adjustment you need to turn the nut labeled #2: tighten to make a sensor work if it doesnt now, loosen if your brake sensor is causing the motor assist to not work now.
Pull lightly on the sensor wire to create tension while you turn the nut. Turn the nut in/out 1/4 turn at a time.
Test the throttle between each adjustment to see if the brake sensor is working. If the throttle spins the rear wheel and pulling the brake causes the motor to stop, then you are good.
If the throttle does not spin the rear wheel, or the sensor does not cut power, then turn another 1/4 turn and test the throttle again.
Once it is adjusted, all that's left is to tighten the set screw, marked 1.