It is managed by Apple. You can view your subscriptions by following the steps:
  1. Open the Settings app.
  2. Tap your name. 
  3. Tap Subscriptions. (If you don't see "Subscriptions," tap "iTunes & App Store" instead. Then tap your Apple ID, tap View Apple ID, sign in, scroll down to Subscriptions, and tap Subscriptions.)
  4. Tap the VPN Clean - subscription that you want to manage.
  5. It will display all information about available subscription plans and active subscription plan (if any) for VPN Clean.

Alternatively, tap here to learn more.