In desktop PC there is a chrome extension to change default video quality for youtube video, but in the phone you can do this even more easily without using any extra app, this will work android phones, windows phone, iphone and other, follow these easy steps :
1. In the phone open and play your favorite youtube video, while playing video tap on video and tap on “three dot” button, now tap on “Quality”
2. You will get some quality like, 144p, 240p, 360p, 720p, 1080p and auto, if you choose “auto” it will automatic adjust video quality as per your internet speed, or you can choose any quality by tapping on that, from next your all youtube video will play in the selected quality.
Watch video tutorial… Please like, share & subscribe