afaik, there is no shift point adjustment on any of these bikes .. 2nd gear activation is controlled by road speed and by spring strength of an internal clutch. I think there's something wrong if it's not downshifting.
Many of these bikes are extremely sensitive to lubrication type and amount.. Check gearbox fluid level and use the recommended fluid type. 350ml of SAE 30 motor oil for the one and two speeds according to this manual break>
more details about that bike's overall condition and it's mechanical history and the history of this particular shifting problem might provide clues to what's up..