1. Log into https://video.rit.edu
  2. Click on “Edit” for the video you wish to change the thumbnail or preview image
  3. Click “Manage Content” tab at the top left of the page
  4. Use the  Options drop-down menu to select “images”
  5. Find an alternative thumbnail or upload a new image
  6. Use the radio buttons to select the desired image at the “Preview” and “Thumb” image
  7. Click Save and Continue, the change will take effect immediately

Detailed steps including screenshots can be found in the following document from the vendor: Ensemble Video -Edit your video's metadata