First and simplest would be to replace the radiator cap. Sounds like it might be getting old and not holding pressure.
The low coolant sensor is in the overflow bottle, hence the light is on but the engine is not overheating.
Try to replace this low fluid with coolant and not just water. The straight water can freeze in the bottle cracking it and over time and refills will dilute the coolant in the motor/radiator.
If you keep having to add coolant after the cap, you need to get the cooling system checked. It has to be going somewhere, lets hope a leaky hose, worst case a bad head gasket.