How to Change Your Thumbnail on a YouTube Video:
1. Sign in to YouTube, if you have not done so.
2. Click your profile image in the upper right hand corner.
3. Click YouTube Studio in the menu.
4. Select the videos tab in the left side menu.
5. Locate your video, and click it.
6. Hit the details tab of the video.
7. In the next page, you will see three options, and a section where it says upload thumbnail.
8. If you want a custom thumbnail, click upload thumbnail.
9. Verify if the new thumbnail is selected, and click save.