I recently changes the media aspects. The problem is that the existing thumbnails are not changed to the new aspects. How to make WordPress regenerate the thumbnails after I changing the media aspects?
The "Regenerate Thumbnails" works like a charm for me: https://wordpress.org/plugins/regenerate-thumbnails/ . It allows you to regenerate your thumbnails after changing the thumbnail sizes.
After installing and activating the plugin in WordPress, go the "Tools -> Regn. Thumbnails" and click the button to start resizing the images.