Originally Posted by AutoSlash

Just call the number on the back of the card and they can take care of it for you. One suggestion though... Instead of calling and asking to downgrade the card straight-away, you might try starting off with asking to cancel the card. The reason is that they will almost certainly offer you some sort of perk to keep the card, and one of these might be just be waiving the annual fee one-time on the Plus card. This way, you can keep all the benefits of the Plus card, without having to pay the fee. If they don't offer to waive the fee or some other similar concession that you find valuable, you can always still cancel then. Nothing ventured, nothing gained as they say.