The top speed of the bikes is electronically limited to 13 mph, as they are configured for trailer operation. This limit can be increased by reprogramming with the Bafang BESST tool, which is commonly available at e-bike shops. You can also contact the US branch of Bafang for support through their website: Bafang USA Support.
Additionally, the speed can be further increased by replacing the front chainring with a larger one. The larger the chainring, the higher the potential speed. Here is a link to a 58-tooth chainring available on Amazon: 58-Tooth Chainring on Amazon.
A larger chainring typically requires a longer chain as well.