
Explanation/Reference:
Note:
* To upgrade a site collection, site collection administrators complete the following steps:
1. Run the site collection health checks to verify the site is ready to upgrade.
2. Create an upgrade evaluation site to preview the differences between versions. (Optional)
3. Upgrade the site collection.
4. Verify that upgrade was successful and the site works as expected.
* After a server farm administrator has upgraded the databases, site collection administrators can upgrade individual site collections. When site collection administrators first browse to their sites after the database has been upgraded, a notification bar at the top of the site indicates that their sites can be upgraded. The choices are to Start now or Remind me later. Start now begins the site collection upgrade process.
Reference:https://technet.microsoft.com/en-us/library/jj219650.aspx