Where can I edit my video information?
After you log in, select 'My Channels' from the account menu drop-down.
Select the Channel name your video is saved under. In the next page, click 'Media' on the left panel.
Click 'Edit' next to the video you want to edit.
Now you can edit the information of your video, including picking a different thumbnail.
Be sure to click 'Update Video' after you make your changes!