When the engine is cold, the level should be at least at the bottom mark on the reservoir. Be careful: when the engine is hot, the coolant automatically expands. There should be enough room in the reservoir for the excess liquid. So do not be misled by the elevated level.
To add fluid, carefully open the tank cap. When the engine is hot, the tank is under pressure. Therefore, place a thick rag on the cap. If you open the cap too quickly, there is a risk of scalding - hot coolant will boil and splash out of the tank.
Addition: If the coolant level in the expansion tank has dropped below the "MIN" mark, fill the reservoir with pre-mixed liquid. When opening the tank, pay attention to the formation of bubbles and oil dirt - this is most often how a defect in the cylinder head gasket is detected
Do not fill the expansion tank above the upper mark. When heated, the coolant expands and flows out of the system.
You can safely add small amounts even when the engine is hot.