If you currently have Fantasy or Sci-fi subscription and want to upgrade to a SuperSyrin, visit the store and log in with your account details and then purchase the new subscription. The store will then calculate the amount that you need to pay to upgrade.
To downgrade from SuperSyrin to one of the fantasy or sci-fi subscriptions, you need to cancel the automatic payment agreement you have set up with PayPal, which then allows your subscription to lapse at the end of the two month period. Once your subscription has expired you can then subscribe at the new lower tier subscription.
Full details on cancelling PayPals automatic payments can be found HERE