If the engine has overheated then the only option is to stop and turn it off. Don't open the radiator cap to refill it until the radiator is no longer hot to the touch. Even then, keep the engine running and the water circulating while you refill the radiator – otherwise you run the risk of cracking the hot metal by suddenly cooling it. Flicking droplets of water on to the outside of a running engine will cool it.
In areas of tall grass keep a close watch on the water temperature gauge. Grass stems and seeds will get caught in the radiator grill and block the flow of air, causing the engine to overheat and the grass to catch fire. You should stop and remove the grass seeds every few kilometres or so, depending on the conditions.