A few bikes have controllers that can run between 36 and 48 volts, but most bikes sold with 36V batteries have controllers that will eventually blow up at the higher voltage. It costs about 50 cents for upgraded parts for a controller last at higher voltage, but that's like 4 bucks in China, enough for a fast food lunch when I was there on a visit, so they don't do it.
If you were able to swap to a 48V 350W controller, around $40-50, that bike could go 20-22 mph with a bigger battery. Electrical experience and a level of mechanical ability is needed to swap.
It wouldn't make sense to buy a $600 bike and then add $450 for battery/controller to go faster. Still, I think that bike, if it holds up, is a nice package. Disk brakes. Front suspension. I know it's all cheap stuff, but still impressive.