Q:“As an admin, how can I replace the thumbnail on a video?”
Assuming you already have the image you'd like to use as the video thumbnail, find the video by going to the Content tab of the admin navigation panel. Click the title of the video, not the existing thumbnail, to go to the video edit page. Under the thumbnail on the right hand side, you'll see a link to "Upload New Image." Click that link and then on the next page, you can browse for the file on your machine, select it, and then upload and save it.
Q:“As a user, how can I replace the thumbnail of a video I've uploaded?”
When you sign in to a channel, the landing page is you account profile page. Click the Show my Videos link in the Content section of your profile. Under the title of each video is a link to Replace Thumbnail Image. Select that link and on the next page, you can browse for the file on your machine, select it, and then upload and save it.