According to the “How to cancel your YouTube TV subscription” article posted at 9to5google.com, you should complete the following procedure:
Log in to your YouTube TV account on the web;
Click your profile in the top right corner, then click Settings;
On the Membership tab, choose ‘Deactivate membership’;
Click ‘Cancel Membership’.
If you cancel during a free trial, you will lose access to YouTube TV immediately upon cancellation.