Go to Site Settings, select 'Upgrade or Renew' and check your current subscription.
If you have an active auto-renewing subscription, you will first need to cancel it at your chosen payment gateway: iTunes / Google Play / PayPal / PayPro Global. You will then be able to subscribe to a Pro site.
The time remaining on your Smart subscription will become Pro
If you have time remaining on your Smart subscription, don't worry - the Pro upgrade will be applied to the rest of your Smart period.
For example, if you have 9 months left on your Smart subscription, upgrading to Pro for 1 year will make your site Pro for 1 year and 9 months.