This is the first time I have heard of an electric scooter's speedometer displaying a faster speed than the scooter was actually traveling at so I do not have any past experiences to draw advice from.
The speed controller sends a speed signal to the speedometer and the controller is what controls the top speed so I do not think that replacing the speedometer would help and the problem is more likely to be with the speed controller or the motor.
There are usually no adjustments that can be made to adjust the top speed or correct the speedometer as these functions are factory programmed into the controller and are not changeable.
I have purchased this scooter https://dyadcycles.com/ca/montreal/en/e500-lithium.html used from a person in my area. The scooter is advertised to do 32km an hour and the dash displays that I am doing 32km/hr. However I felt it was a bit slower than that and after testing next to my dads ebike and his car the scooter is actually doing 23-24km/hr. In addition to that just walking with the scooter while its on displays 9km/hr. I was thinking maybe there was a problem with the speedometer which was limiting the power to the motor to maintain the standard 32km/hr? Any ideas? I plan to bring it in to get checked out but I dont really want to spend $100 on a checkup and need to pay an addition $500 for a new motor or something when I spent like $800 on the bike unless its just a minor adjustment that needs to be made. Thank you for any and all help!