Go to Site Settings and select 'Upgrade or Renew' and check your current subscription.
Before upgrading from Smart to Pro, you should make sure that there is no auto-renewal subscription on your Smart website. If there is one, you will need to cancel it by logging in to your account on the corresponding payment gateway (iTunes / Google Play / PayPal / PayPro Global).
Once you do not have an active Smart subscription, you will be able to opt to subscribe to a Pro site.