FluentU subscriptions automatically renew at the end of each billing period. You are notified each time there is a charge to your credit card.
If you would like it to not automatically renew, then you should downgrade to Free. You would continue to have access until the end of the current billing period.
Since you subscribed through iTunes, you'll need to unsubscribe through the settings on your device. Here's how to do that:
First: go to Settings on your device and choose iTunes and App Store from the left-hand side. Then touch the Apple ID in the upper right-hand corner (see the arrow).
The system will ask you to sign in to your iTunes account, so go ahead and enter your password.
On the next screen, touch View Apple ID.
The next screen that pops up says "Account Settings." Scroll down a bit and you'll find "Subscriptions." Touch this, and you'll see all your subscriptions through iTunes.
From there, you can manage your Subscriptions. You should find the Cancel Autorenewal menu item there.
I hope this helps!