If your bike has the square taper type of crankset then it's somewhat easy. I just changed the crankset on a friends voltbike from 175 I think it was to 152mm. His reason was a bit different, he was experiencing pedal strike trying to follow me on the trails. Pedal strike is when either side pedal on the downstroke hits a rock or stump, etc. He still has to pay attention but the smaller crank arms give better clearance. Purchased from Amazon, just search square taper cranksets. His bike came with a 44 tooth chainring but all I could find was in a single chainring was a 38 tooth.
The 38 helped as well making it easier for him to start off pedaling without needing the throttle.