Zune.net will be updated tomorrow at 10:00 AM (PDT) to add account management directly from the site without the need to log in using the Zune Software... 

Tomorrow, around 10 AM PDT (1 PM EST, 6 PM GMT), the support page on Zune.net will update, giving you the ability to manage your account from a web browser - no need to sign into the client. This will enable a few things, like:

• Update, change, or cancel your Zune Pass subscription
• Buy Points
• Change the password on your Tag
• Update your credit card info, view your statement

[via ZuneInsider] 

EDIT: This is the new interface in Zune Support page:

After signing in your Microsoft Passport (if not already) you can use any of these options. Check the billing system when you click on "Account Management" :