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But believe me when I say the new OS will wait for you and making sure your current data is safely backed up isn't something to overlook.
At this point, the El Capitan installation has finished and is displaying the OS X Login screen.
Most major system updates include changes to Apple apps, and it's likely that when you first run apps, such as Mail or Photos, the app itself will update associated user data.
In the case of Mail, your mail database may be updated.
Apple once again provides only two basic installation methods: the upgrade install, which is the process this guide will take you through, and a clean install.
Upgrade Install overwrites your current version of OS X, replaces any outdated system files, installs new system files, resets file permissions, updates Apple-supplied apps, and installs new Apple apps.