Here's how to downgrade to free if you upgraded with the in-app-purchase:
1. Go to Settings on your device
2. Find "App and iTunes Store" and tap it
3. Find your Apple ID and tap it
4. In the "Apple ID" popup, select "View Apple ID"
5. Find FluentU payment under Manage App Subscriptions along with the option to turn Auto-Renewal off (if you don't find FluentU payment, try another Apple ID)
6. Turn Auto-Renewal off