by Bret Miller.
Especially if you have a site with a lot of data, I would suggest using the command-line upgrade tool if at all possible. You get more information about what it happening and it's not subject to execution time limits like the website is.
php admin/cli/upgrade.php --non-interactive
Run that from the moodle public html directory on your server. If you have multiple versions of PHP, you may need to specify the path to php to get the right version.