I am a new owner of a 15-year-old home. Each room has two air returns--one near the ceiling and one near the floor, with the heating/cooling vents under windows where you commonly find them. I want to make sure I'm managing the returns correctly. Now that it is AC season, should I be opening up the returns near the ceiling to allow the rising hot air back into the system, while closing down those near the floor to keep our cooled air in the room? And vice versa for winter? Thanks for your help.

Yes high in the summer and low in the winter. Or if you do not want to change them all the time just leave them all open. Inless you have very high cellings it is not going to make that big of difference.

Yup.