You don't have to. They'll just deduct it from the next thing you charge. Or if for some reason you cancel the hard and it has a positive balance, they'll mail you a check.
As Cat indicated, they will just deduct it from future charges. I get that each month with my Walmart card since they give a 1% cashback statement credit each month. Since I don't take the time to figure out what the credit will be, I end up overpaying each month.