Guide

1. Go to the left-side panel and click 'Content'.

2. After that, hover your mouse cursor over the video you want to edit and click its corresponding 'Details' button.

3. Under the Thumbnail section, click 'Upload File' and select the image file you want to use.

4. Once done, click 'Save' in the top-right corner.

5. After completing these steps, the thumbnail of the selected video will be changed based on your input.