I think if you check your Steemit blog, you'll find the post of your video and you can edit the description and tags if you want. I was able to edit the tags on one of my videos after it was published.
I wish I could say the same for thumbnails, but I don't see a way to do that yet.
I guess I'll have to try it out again and see what's up. Here's a link to the original post, check it out! It's a film we streamed with some video art and music a few days ago! Enjoy.
Actually, I just checked again and any edit you make on a Steemit post will not show up on your DTube video. So it looks like we have to wait until DTube implements an edit button.