We eventually will have a way for you to update your credit card information through your account settings. However, right now you simply need to turn off your auto renewal and subscribe again as an existing user by going to on3.com/join.
To turn off your auto renewal, head to https://www.on3.com/my/settings/ and hit "cancel subscription." That doesn't actually end your membership. It turns off auto renewal. Your account remains active through your billing period.