DISABLE wordpress upgrade page

You can actually do this on the web host without actually any PHP code. Using the same procedure as is recommended to deny access to wp-config.php, you can also deny access to upgrade.php.

All you need to do is create a file named .htaccess in wp-admin and put the following in it:

<files upgrade.php>
order allow,deny
deny from all
</files>

What this does is it tells the web server software do deny access to upgrade.php from everybody. That way if someone tries to access it, the server will return a 403 error instead.

This assumes your web host is running Apache as its web server software. If it is not, their support staff should be able to assist you.

File not found.