You can now start the installer by launching the Install OS X El Capitan app in the /Applications folder on your Mac.At this point, the El Capitan installation has finished and is displaying the OS X Login screen.OS X El Capitan can be found in the Mac App Store as a free upgrade for anyone who is running OS X Snow Leopard or later.
This means if you start downloading the El Capitan installer from the Mac App Store, and get up to have some tea when you come back, it's very likely that you'll be looking at the El Capitan installer screen waiting for you to click the Continue button.
Snow Leopard is the oldest version of OS X that can access the Mac App Store.
Create a Bootable OS X El Capitan Installer on a USB Flash Drive This step is optional but can be helpful if you have multiple Macs to update because you can use the bootable USB flash drive to run the installer from, instead of downloading the OS from the Mac App Store on each and every Mac you intend to update.
There are quite a few more steps involved in the update process, but the one thing an upgrade install won’t do is change any of your user data.
Even though the installer doesn't touch your user data, that doesn't mean the data won’t be changed soon.