that radiator plug can be a little confusing, insert a 1/4 extension in the center and turn carfully, it has to turn and come out towards you at the same time, gm has flush procedures for dexcool, but dexcool is good stuff regular antifreeze become acidic after years and eats metals and gaskets dexcool usually doesnt eat metals but it does turn to chunks if air gets to it the good thing is it flushes out easily, if you still have green antifreeze in your car get out your meter and put one lead to battery negative, the other in the radiator
if its turning to acid it will make .1-.2 volts time to change that stuff. (no kidding i am mr. goodwrench lol)