*Accounts created after February 1st, 2016 can change their site plan from the admin dashboard. Read this article to find out how.
If you want to make a change to your subscription plan, you will first need to cancel your current subscription.
This does not remove your site and this will not cause your site to revert to ad-supported because your subscription will still have remaining time. So long as you establish a new subscription before that time runs out there will be no changes to your site whatsoever.
After you have canceled the current subscription, you can set up a new subscription on the Admin Menu -> Billing -> Setup Subscription area of your site. Any necessary prorating will be done during the order process and your site will be upgraded in about 10 minutes.