If you are getting the yoursite.com/wp-admin/upgrade.php?_wp_http_referer=%2Fwp-admin%2F error, this worked for me: Navigate via FTP to your installation. Check to see if the upgrade.php file is there. If not, download WordPress 3.4.2 from https://www.www.remarpro.com. Unzip the download and navigate to the wp-admin folder. Locate the upgrade.php file and FTP it to your installation. Somehow the in-app update process download was corrupted and missed the required upgrade.php which the redirect points to after the update. Good luck!
