They do not support changing subscriptions directly, you can only unsubscribe the current one and re-subscribe the new one.
You may manage your premium subscription in the device settings:
Go to settings
1. iTunes & App Store
2. Tap your Apple ID at the top of the screen
3. Tap View Apple ID. You might need to sign in or use Touch ID
4. Tap Subscriptions.
5. Tap the subscription that you want to manage.
Then you can see your current subscription details, including price and kind.
You can choose another subscription plan according to your needs.
The new package will take effect immediately. The new subscription payment will be deducted immediately, and a refund will be automatically made according to the proportion of unused subscription duration.
Please note: If the status is shown as free, please try to restore it.
After the current subscription expires, the new subscription takes effect, and then the new subscription price will be deducted.