How to get refunds on transit card?

We will visit Shanghai and expect to make some use of the subway -- how much, we're not sure. If we load more onto the card than we turn out to need, can it be refunded? How do we get our deposit back?

We will purchase our cards somewhere near Baoshan, or possibly at People's Square; we will use them for the last time several days later to travel to Pudong Airport.