Hot air rises and when the outside temperature is hot you must adjust your vents to force the cool air to the upper floors. You will need to adjust you vents in the summer months in order to keep the upstairs cool. This can be accomplished by adjusting both the upstairs vents and the downstairs vents.
Second Floor Vents
Locate all of the vents on the second floor and check to see if they are blocked in any way.
Remove any furniture blocking the air flow.
Fully open each vent to allow for maximum air flow.
Find each vent and close the register half way.
Ensure that the air conditioning unit is on and wait one hour.
Check the temperature of the second floor rooms. If they still are warmer then the downstairs rooms, close the downstairs vents more. Repeat until you are satisfied with the temperature of the rooms.