Edit Image Thumbnails Separately
Edit uploaded WordPress images with the option to edit thumbnail size only or all image sizes except for the thumbnail size.
Edit the Image Thumbnail size or bypass it
The option to edit image thumbnails separately has been removed from WordPress 6.3 onwards. You can enable this option with the Edit Image Thumbnails Separately plugin from the WordPress Core Contributors.

You can see the before and after in the above screenshot. This plugin restores the image editing ability for thumbnails only or all sizes except thumbnails.