Cal here from Shopify, thanks for reaching out to us. How are you keeping?
You can change your selected any time by going to Settings > Plan and Permissions and choosing whichever plan you wish. You can find a comprehensive breakdown of each plan and its features here. If you are looking to cancel your store before that date you would also go to that same page in order to do so. Let me know if you are having any further trouble with that.
How has the store setup gone so far? Have you enjoyed your trial? Have you encountered any stumbling blocks so far?