A thumbnail is an image with a reduced file size that is used as a placeholder for full sized multimedia content. Follow the below steps to make the change on the editor
Step 1. On the Editor, Click Produce
Step 2. Click Settings tab
Step 3. Click Upload to add an image from your computer.
Step 4. Resize and drag accordingly.
Step 5. Click Apply when you are done.
Step 6. Use the Image editor to make changes as you see fit or just click Apply to proceed. Click here for an article on Image editor.
Step 7. The thumbnail has changed to the uploaded image.
That's it. You're done 😃