A free WordPress plugin named Regenerate Thumbnail will help you to easily regenerate and upload your thumbnail image.

Follow the steps below to regenerate the WordPress thumbnail.

1. Install and activate the Regenerate thumbnails plugin.
   
   
2. Next, go to Tools > Regenerate Thumbnails. Here you will find settings related to WordPress images.    
   
   
   
   
3. To regenerate all the images click on the Regenerate all thumbnails button. 
   
    
   
   
4. After you press the button, all the images will be defined by their new thumbnail size that is given in your theme or settings > media.
   
   
5. You can also regenerate images selectively, by going into Media > Library and then clicking on the regenerate thumbnail button next to a post.