(didnt read other responses)

u dont need to remove the whole undertray. if u look, there is a small panel in the front and slightly towards the passenger side. that panel uncovers a small black plastic nut on the bottom of the radiator. loosen the nut and radiator fluid drains.....

i dont feel u need to get to the engine drain plug also if u follow a cleaning procedure in between.... case u dont know the correct procedures of radiator flush, do research on google.