You may manage your Premium subscription for iOS through your App Store account settings. Please take the following steps to manage your subscription:
- Go to Settings >> iTunes & App Store.
- Tap your Apple ID on top of the screen
- Tap ‘View Apple ID’. You might need to sign in use your password or Touch ID.
- Tap Subscriptions.
- Tap the subscription that you want to manage.
- Use the options to manage your subscription
Click here for more details: https://support.apple.com/en-us/HT202039