Video quality doesn't change to begin with

When I change the video quality to a better one in a good wi-fi, and then skip to the start of the video, it doesn't change from begin with. That sucks, because I need a screencap of something in high quality, but it's in the first half of the video where even if the quality is changed, it's still at 720p. It always happens and I wonder if there's a way to fix it. To get the whole video in 1080p.