You cannot cancel your account from the Paramount+ app if you signed up using a web browser. To manage your Paramount+ plan, you must return to the platform where you originally created your account. If you did you the iOS app, then tap More > Account and follow the instructions to cancel. You can also open the Settings app and tap your Apple ID. From here, tap Subscriptions and select Paramount+ and use the options to cancel your subscription.
How do I cancel my Paramount+ account?