The App Store sells exams, bought once, and not a Premium subscription, so there is nothing to cancel or renew.
In the app you can buy an exam through the App Store. It is a one-off purchase, never a subscription. Like a purchase on licento.com, it gives you the exam for good and comes with the same period of Premium, which ends by itself.
A Premium subscription is sold on licento.com, under your account, and is managed there.
Your App Store purchase belongs to the Licento account you were signed in to when you bought it. On a new phone, or after reinstalling the app, sign in with that account and tap «Restore purchases» in Me. A purchase made under another account stays with that account.
Refunds for App Store purchases are Apple's. If Apple refunds one, your access to that exam ends.