There’s no way in the WP admin area to replace existing media files in situ. You could replace existing files with the exact same name via FTP. But in the case of image files, you’d need to also replace all the various generated sizes with updated versions of the same size.
The alternative is to upload the new version as a new media file and then alter any posts using the file to use the new version instead. The media library reference to the post a media file is attached to is unreliable for knowing which posts the file is used in. Instead, search the posts DB table through the phpMyadmin app to find posts having that file name in post_content. You can instead use the WP posts list table search box, but you need to search separately for every post type. You can see all post types found at once with phpMyAdmin.
