How to Manually Upgrade WordPress Software
To manually upgrade the version of WordPress you have, first make sure that your website is backed up. Then, go to WordPress.org and simply download the latest version of the software. Once you’ve downloaded it, extract the zip file to your computer. Next, open the file that contains all of the WordPress files and delete these items:
The wp-config-sample.php file
The wp-config.php file (if this file is there, which it might not be)
And the wp-content folder
After you delete these, upload the remaining files and folders to your server via FTP. When you are prompted about duplicate files by your FTP client, tell it to overwrite all files. Once the file uploads are complete, your WordPress site will be running on the most recent version, and the error should be resolved! Don’t forget to also update all of your WordPress themes and WordPress plugins to the latest versions, too.