You can just do the equivalent of a blue-green deployment: you have two copies of the firmware, a flag that tells you which is active, and a tiny bootloader that respects that flag on startup. At any point during regular operation the firmware can update the other copy, and once done toggle the flag. Next time the scooter is started it uses the new version. If the device is shut down during update it continues to boot from the old version, and just tries again next time. Bonus points if you have a small watchdog that toggles the flag if the current version fails to boot.

One pretty understandable reason for a transportation device to update itself is for new GPS ephemeris data. I think it would be cool if there was a device protocol that somehow securely promulgated such data from devices that are expected to be well-connected, like your smartphone, to "things" that are not, like watches and cameras and scooters.