To edit your videos, login to Pornhub: http://www.pornhub.com/login and go to the Video Manager: http://www.pornhub.com/video/manage. You can set the visibility of your videos to public or private from here as well as select a thumbnail image.
Unfortunately though, once the video has been uploaded, the title, tags and categories cannot be changed. You would have to remove it and re-upload it with the new information.
Note that this feature is only available for verified accounts. Get Verified!