Updates can fail for a large number of reasons and it is often hard to diagnose the failure due to specific configurations present on each server running the update. For example, permissions may be incorrectly set, the unpacking of the new version might not have been properly verified due to high system loads with your host if many users are updating at the same time or they have a very loaded service, An IO hang, files are being validity checked and they fail due to a corrupted bit. There are many more reasons that things could have failed. Honestly there’s no good way to track down why a partial update occurred, we can only recommend what to do if one does occur:
Try MANUALLY updating. Download WordPress again and unzip it, and use that to copy up. Access your server via SFTP or FTP, or a file manager in your hosting account’s control panel (consult your hosting provider’s documentation for specifics on these), and delete then replace your copies of everything on the server EXCEPT the wp-config.php file and the /wp-content/ directory with fresh copies from the download. This will effectively replace all of your core files without damaging your content and settings. (NOTE: do not delete the wp-content directory or your wp-config.php file). Please read the Manual Update directions first.
Backup: If you haven’t already done, always backup everything (including your database) before doing any actions, just in case something really goes wrong. You can never have enough backups! See https:
