DiroDi Primo comes with the Shimano Nexus 3-speed internal hub gear. To change the gear, you should twist the gear shifter on the right side of the handlebars. Just like gears in cars, you should start with gear #1 and then change to higher gears if you want to go faster. To climb hills, you should put the bike on gear #1.
Please refer to the short video below on how the hub should be adjusted
TIP: If the adjustment screw is too tight to be turned by hand, please use a plier to turn the screw. The locking nut should be loosened using a wrench size 10.