Despite the small size, the KD21C has a wealth of features.
Link to owner's manual:
https://support.ride1up.com/helpdesk/attachments/65003445148
Chapters:
0:00 Quick Start Guide
1:15 Settings Menu
2:00 Factory Reset
2:12 Wheel Size, Top Speed
2:30 Advanced Settings
3:47 Voltage Readout
3:59 Pedal Assist Customization
4:42 Password Setup