How to change a NumberValue's value?

I want to change a NumberValue's value whenever the car goes backwards and forwards.

The NumberValue

local MS = seat.Speed.Value

The function that has to change the NumberValue's value.

local function onChanged(property)
	if property == "Throttle" then
		LD.AngularVelocity = MS * seat.Throttle
		RD.AngularVelocity = -MS * seat.Throttle
		if seat.Throttle >= 0 then
			MS = 77
			print("going forward")
		else
			MS = 30
			print("going backward")
		end
	end
	if property == "Steer" then
		LS.TargetAngle = SA * seat.Steer
		RS.TargetAngle = SA * seat.Steer
	end
end
seat.Changed:Connect(onChanged)