Hello,
I saw the screenshot you attached. To change the image thumbnail you need to change the product image from edit product.
Screenshot for Clarification: https://ibb.co/QN5GN17
You need to view your product image thumbnail to get the path.
Or the second option is you need to check which image is used as the placeholder and then you can replace that image with the same name as it used before.
Screenshot for Clarification: https://ibb.co/b7Y1mgN
Note: You need to upload the image of the same name to replace the file from the path.
Best Regards.