Even when your video is published you can change the title of your video. You go in "Publications', then click on the pen to modify it. Choose the right channel that contain your video. On this page, you can change the name of your video.
However, you have to be careful, when you change the name of your video this may cause a decrease of your SEO. You really have to be sure before changing the name of your video.
Finally you save the modification in order to see the new title on your video.