To upgrade or downgrade your subscription plan, you can go to the Home -> Settings (gear icon) -> Manage Team Subscription -> "Update Subscription Tier" on your iPhone or iPad.
To cancel your subscription, you can go to the Home -> Settings (gear icon) -> Manage Team Subscription -> "Manage Subscription" page on your iPhone or iPad.
Note for Mac users: Teams subscriptions cannot be changed or cancelled from the Mac app.
This will redirect you to the App Store page where you can cancel the subscription.
You can also manage your subscriptions directly through Apple. Read this article for more details.