So ...

In order to quickly set, replace or remove featured images, you'll have go to Featured Images => Bulk Edit. This is where you need to define the actions and changes that will be implemented according to your needs. It's a multi-step process that comes with an easy-to-follow progress bar:

On the first page you can define actions for a single image, for multiple images or you can proceed without selecting any images. And here are your options:

• actions with a single image
  • set the selected image as new featured image = you can select a single image as featured image for all your published posts (will also replace existing featured images)
  • replace featured images by the selected image = will replace the existing featured images (no actions regarding future posts)
  • remove the selected image as featured image = looks for the image on your website and removes it as featured image
    
• actions with multiple images = you can upload multiple images to use them as randomly featured images on all your existing posts (I love this feature!)
• actions without any selected image = you can remove any image as featured image or you can remove all your existing featured images without replacing them with new image files
  

When you are happy with your choices, simply click the "Next" button and customize the following two options (steps) according to your needs (Add filter & Refine). Finally, you'll be taken to the Confirm and/or Perform tabs (depending on your previous choices) where you can actually apply the changes.

NOTE: you can not undo these changes! It's higly recommended to create a full website backup before performing any changes!

OK. Now let's see the second function ...