You can't change your video after uploading for the same reason you can't change your submission on websites like Reddit after submitting it. It would be too easy to bait-and-switch. I can understand that it's frustrating for honest uploaders but that makes total sense to me.

In general people just mention that there's an updated video in the title or description and you can redirect yourself there if you want. A bit lo-tech but it works well in practice IMO.