Make sure you have
YouTube® app version 12.44 or higher installed on your phone.
While playing a video in the
YouTube app, slide two fingers apart to expand the video to 18:9 ratio.
Slide two fingers together to change the video back to its original ratio.
Note: Expanding the video to 18:9 doesn't resize it to fit into the screen size. Subtitles may appear truncated if they're too close to the bottom edge of the video.