To change the quality for a video, you must be subscribed to the performer. Once you choose a video, click the number icon at the bottom of the video (usually 540 or 1080) to select the menu and change the quality.
Once you change the video quality for a video, the system should automatically use the same video quality for future videos.
Please note that if the video was uploaded by the model in a lower resolution, increasing the quality on your side will only help so much.