Since it is a 48 Volt electric scooter it will be relatively simple to increase the top speed because the batteries and charger will not need to be replaced.
To increase the scooter's top speed a motor and speed controller with a higher Watts rating, like 750 or 1000 Watts, could be installed. Then the gear ratio could be increased by installing a smaller sprocket on the rear wheel.
That is all there is to it. When upgrading from a 500 Watt motor and controller, to a 750 or 1000 Watt motor and controller, the wiring between the batteries and controller will also need to be upgraded to a higher gauge wire than original wires so they can handle the extra power that will be going through them.