After creating a Campaign, the Campaign Manager screen will show your Campaign Thumbnail.
Hovering over the Campaign Thumbnail will reveal a more options icon (3 dots).
Clicking on this will reveal a few options, including Edit Campaign.
The Edit Campaign link takes you to this screen, where you can update your Campaign Title and Video Link.
Note: If you change the video of an active Campaign, you will lose all results related to the original video. If you wish to retain previous results, create a new Campaign instead.
You can also use the Edit Campaign link within the Campaign Results screen. Just click on the option in the Title Toolbar.