If you need to add water to your car's radiator, here are some tips:
1. Use distilled water if possible. Distilled water is free of minerals and impurities that could damage the cooling system.
2. Only add enough water to bring the coolant level up to the "full" mark on the radiator reservoir.
3. Drive the car for a short while to mix the water and coolant.
4. As soon as possible, get your car to a mechanic to have the coolant replaced.
It's also important to note that you should never add water to a hot engine. This could cause the engine to crack or warp. If the engine is hot, let it cool down before adding water.