Checking coolant level under the radiator cap must be done on a cold engine.
If any pressure in the system, it can quickly be released and cause bodily harm.
Press down and turn past the 2 steps in the cap. If cap is on the reservoir, just twist it off.
Sometimes they are a little hard, so may need quite a but of force to remove. Either style cap is turned counter clockwise to remove.
If coolant is low, have the system pressure tested and checked for leaks.
How To Check Your Engines Coolant