From the composer, select the video you'd like to publish.

Now click on the uploaded video

Click on Edit Thumbnail
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Now select if you want to upload an image as the custom thumbnail or select from the video frame

After selecting your option, click on the APPLY THUMBNAIL button. Then CLOSE
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You'll see that your post preview will now display the new cover image selected/uploaded.
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You can proceed to schedule/publish your post.

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