You can change the front chainring. FSA makes them from 36 to 46t. Others may make them as well.
edit: It looks like there are few different systems. Some have gearing such that the front chainring is not directly connected to the crank. I've seen mention of a 2.5x gear ratio so a 15t front chainring would be equivalent to a 37t chainring on a regular bike. Bosch's documentation on these details seems a little sparse.
Last edited by gregf83; 07-20-19 at 09:25 AM.