Under the username for the subscription you want to cancel, find the "next billing" heading, then click the Turn off auto-renewal link. You'll get a message trying to convince you not to cancel. Click ...
The process for canceling your YouTube TV subscription is the same whether you're doing it inside the YouTube TV app or from a web or mobile browser, provided you're paying Google directly for access.
I've noticed just how many subscriptions sneak into my budget without me really thinking about them. It's easy to lose track ...