If you sign in at http://billing.microsoft.com with the LiveID for your subscription, you should be able to click the link to renew and provide payment information or a new Retail PIN, if you pick up a retail copy.
If you renew this way, it can take up to 24 hours for the renewal to be reflected in OneCare.
Alternatively, you can reinstall and activate again with a new LiveID. I believe that using the same LiveID will allow you to then provide payment information or a key since the current subscription is expired.
What do you mean by "change my billing method?"
-steve