For Regular accounts: 

Regular 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 at least one day before the renewal date. You would continue to have access until the end of the current billing period.

You can downgrade your account to Free by going to your account settings (click on "More" > "Settings" on the left panel) and then the Subscription section.

For Academic Plus accounts:

Academic Plus accounts do not automatically renew. If you'd like to continue your membership after the first year, go to Settings > Subscription and click "Renew subscription".