Change my subscription
You can do so by going to the "My Account" section of the Android app and tapping the top preference. This will walk you through the selection process again.
Since all subscriptions are managed by Google, we cannot cancel your old one for you. When upgrading, you will be redirected to the Play Store to cancel the old subscription.
Cancel my subscription
If you have a subscription, you will need to go to the Play Store page to cancel that subscription, since they are managed by Google.