E420,The HVAC is not cooling the air in the car. Fan works, but no cooling. Can this be fixed by YourMechanic?

The HVAC system is not cooling the air in the car. Fan works. Heat works. Cooling does not work. Can Your Mechanics fix this or must I take it to a dealership. I suspect it needs an injection of refrigerant.

My car has 285000 miles.
My car has an automatic transmission.