How to Update WordPress to 6 Manually.

Summa’s notes

1, You may need to stop the httpd service first if you are using a non-hosting platform.

For example,

I am using Centos Stream, to stop the web service,

systemctl stop httpd.service

When the update is finished, to start the httpd service,

systemctl start httpd.service

2, I skipped the steps for deactivating and activating plugins.

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These are the short instructions, if you want more check out the extended upgrade instructions. If you experience problems with the Three Step Update, you may want to review the more detailed upgrade instructions.

For these instructions, it is assumed that your blog’s URL is http://example.com/wordpress/.

Step 1: Replace WordPress files

Get the latest WordPress zip (or tar.gz) file.
Unpack the zip file that you downloaded.
Deactivate plugins.
Delete the old wp-includes and wp-admin directories on your web host (through your FTP or shell access).
Using FTP or your shell access, upload the new wp-includes and wp-admin directories to your web host, in place of the previously deleted directories.
Upload the individual files from the new wp-content folder to your existing wp-content folder, overwriting existing files. Do NOT delete your existing wp-content folder. Do NOT delete any files or folders in your existing wp-content directory (except for the one being overwritten by new files).
Upload all new loose files from the root directory of the new version to your existing WordPress root directory.

NOTE – you should replace all the old WordPress files with the new ones in the wp-includes and wp-admin directories and sub-directories, and in the root directory (such as index.php, wp-login.php and so on). Don’t worry – your wp-config.php will be safe.

Be careful when you come to copying the wp-content directory. You should make sure that you only copy the files from inside this directory, rather than replacing your entire wp-content directory. This is where your themes and plugins live, so you will want to keep them. If you have customized the default or classic themes without renaming them, make sure not to overwrite those files, otherwise you will lose your changes. (Though you might want to compare them for new features or fixes..)

Lastly you should take a look at the wp-config-sample.php file, to see if any new settings have been introduced that you might want to add to your own wp-config.php.

Step 1.5: Remove .maintenance file

If you’re upgrading manually after a failed auto-upgrade, delete the file .maintenance from your WordPress directory using FTP. This will remove the “failed update” nag message.

Step 2: Update your installation

Visit your main WordPress admin page at /wp-admin. You may be asked to login again. If a database upgrade is necessary at this point, WordPress will detect it and give you a link to a URL like http://example.com/wordpress/wp-admin/upgrade.php. Follow that link and follow the instructions. This will update your database to be compatible with the latest code. You should do this as soon as possible after step 1.

Don’t forget to reactivate plugins!

Step 3: Do something nice for yourself

If you have caching enabled, clear the cache at this point so the changes will go live immediately. Otherwise, visitors to your site (including you) will continue to see the old version (until the cache updates).

Your WordPress installation is successfully updated. That’s as simple as we can make it without Updating WordPress Using Subversion.

Consider rewarding yourself with a blog post about the update, reading that book or article you’ve been putting off, or simply sitting back for a few moments and letting the world pass you by.

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Final Steps

Your update is now complete, so you can go in and enable your Plugins again.
If you have issues with logging in, try clearing cookies in your browser.

Ref: https://wordpress.org/support/article/updating-wordpress/