We have a lower and an upper air return vent for heating and a/c. how should they be postioned for the different seasons?

In the summer you want to take the return from the high part of the room, this is where the warm air will tend to be. In the winter you want to take the return from the lower area of the room, this is where the cooler air will be and the warmer air is up higher and you want to leave the warm air in the room.
If you have a ceiling fan it would also help if you would reverse the fan in the winter time and try to blow some of the warm air away from the ceiling and yet not cause a wind chill effect, so have it on low speed.

Let's modify this a bit. "take" = "remove the air from"

Cold air is denser so it's found at the floor.
Hot air is found near the ceiling.

In the winter, you want to "remove the cold air and heat it" so floor return is open.
In the summer, you want to "remove the warm air and cool it", so ceiling return is open.

Same as what letmetellu said.