To go faster, you'd change the front sprocket to a larger one, or the rear sprocket to a smaller one. To figure out what new one to use, determine the ratio between the speed you want and the speed you have now, and then multiply that ratio times the number of teeth on the rear sprocket (or divide it for the front sprocket).
However...it will then suffer slower and less powerful startups because you lose torque by the same ratio you gain speed.
Also, the actual speed you get out of it will depend on the amount of power it takes to overcome the wind resistance at that speed, under the conditions of that path/road at that time (weather, pavement, terrain, etc).
If it doesn't go as fast as you expect, you would need to upgrade the controller to higher power, which may require a bigger battery and motor as well, depending on their capability.