The Upgrade Networks screen is used to upgrade all the sites in a Network after a WordPress upgrade is completed. After a WordPress upgrade, you are reminded to visit the Upgrade Networks with a message such as, "Thank you for Updating! Please visit the Upgrade Network page to upgrade all of your sites."
The Upgrade Networks feature will step through each site, five at a time, and make sure any database changes are applied. This menu item is only visible if you are logged in as a Network Admin. You can access it from any site in the network.
If for any reason a site does not get upgraded, each site should be upgraded when the admin for that site logs in to the administration for that site. Sites that have been deactived will not be upgraded by this process, however, if a site is reactivated, the site will get upgraded when an admin for that site logs in to the dashboard for that site.
You can upgrade all the sites on your network through this page. It works by calling the upgrade script of each site automatically.