I don't think you can renew a discounted AP online. You can renew in person if you'll be at WDW within 30 days of your current expiration or you can call
DVC for a renewal voucher or will call number. And any renewal you may order via phone would also have to be redeemed no longer than 30 days after your original expiration, as ticketing needs to see your DVC Membership card. Otherwise you'll need to pay the non-renewal rates.