To update a video's metadata or thumbnail first go to the Video search
Search for the video you wish to update
Click into the video to the VIDEO INFO page
Update any fields as desired
This can also be done on the ADDITIONAL or DISTRIBUTION tab
To update the thumbnail scroll down to ASSETS<THUMBNAIL FILE<UPLOAD
Select a new thumbnail and crop as desired and save your new image.
Once you have made all changes necessary select publish on the top right-hand side- This can be done from any tab.
You will then get a success message and the status will be updated to Processing
Once the update has completed the status of the video will change back to Public (or Unreleased if the video is not yet live)
You can use the CHANGE LOG tab to check that the fields you have changed have been updated as you expected.