If you have an existing subscription that you signed up for on your iPhone or iPad, and you wish to upgrade or downgrade to different subscription plan, follow these steps:

1. Go to your device's Settings. 

2. Click on the Profile option at the top of the screen.

3. Select iTunes & App Stores.

4. Click on your Apple ID.

5. Choose View Apple ID.

6. Scroll down and select Subscriptions.

7. Find and tap on the app.

8. Choose your new plan.