Remove the radiator cap while engine is cold then start your car, let the coolant (anitfreeze) warm up, turn on the heater and watch the coolant level.
What you are doing is allowing the coolant to circulate through the engine without any backpressure, and it will allow the air to "burp" out and the coolant level should drop allowing you to top it up with additional coolant or hot water (not cold)
Sometimes helps if your car is facing uphill while doing this.
Bob Bailey, January 2009