iTunes Movies and TV Shows settings
In Settings , go to Apps > iTunes Movies and TV Shows to change these and other settings.
Change the video resolution for purchases and rentals. By default, you rent, purchase, and play back iTunes videos in 1080p, which requires a fast Internet connection. If you have a slower connection, select Video Resolution and choose a lower resolution.
Change the video resolution for previews. By default, iTunes previews are high definition (HD). To view standard definition (SD) previews, choose Standard Definition.