How do I change video quality or turn off autoplay?

Devices → Playback sets video quality, autoplay of the next episode, and previews.

Go to Account Dashboard → Devices → Playback.

Video quality: Auto, High, or Data saver (about a third of the data).
Autoplay the next episode: off means the next episode is offered but doesn't start on its own.
Autoplay previews while browsing: off stops the silent trailers that play as you browse.