You will find your current plan and the expiration date under Account Settings when you click the small profile icon in the top right corner, when logged in.
To change your plan: Click 'View Available Plans' to see your options. If you had an expiration date it would appear here. If you are subscribed to recurring billing and did not make a one-off payment, you can cancel the recurring billing in this section.
If you don't see your Pro or Lite subscription here, you may need to log out and try other emails you might have registered with Piktochart to see if your Pro or Lite plan is in another account instead. This can happen with some users who have more than one email registered to Piktochart :)