When uploading a video, you'll only be able to change the thumbnail as you initially upload it.
First, navigate to the Content Library, and select Create Content.
Once at the Create page, click 'Upload Video', and add your video.
Then you'll see a 2nd window of the video appear under the word Thumbnail (shown below).
To select the thumbnail, drag the time slider from the top window to the desired image. Once you've done so, click the TV icon next to Thumbnail and you'll see the bottom window match the top window.
You've now selected your thumbnail! Now you can finish creating your post and click Save Content.