If you have access to install, or if it is available on your server I would recommend using WP-CLI. It has a command to regenerate the media:
wp media regenerate
If not, you can try a plugin such as Regenerate Thumbnails
Good luck!
If you have access to install, or if it is available on your server I would recommend using WP-CLI. It has a command to regenerate the media:
wp media regenerate
If not, you can try a plugin such as Regenerate Thumbnails
Good luck!