El Capitan is a versatile operating system version of Macintosh. Many people improve and downgrade to El Capitan because of the beautiful features. This is the new OS X family update. El Capitan Download features include a new device font, Mission Control Window Manager Updates, split screen view, improvements in Spotlight Quest, upgrades to Mac applications such as Safari, Mail, Notes, Images, Charts, etc. Any Mac machine or laptop that works in OS X Yosemite or OS X Mavericks can be downloaded directly from El Capitan Page. Perhaps this is the perfect time to try this awesome Mac OS release, if you haven't tried it before. You can downgrade your Mac OS to El Capitan at any time you like if you are using Mac OS Sierra and High Sierra and are more pleased with the previous version of El Capitan.
That means that El Capitan is mounted on an empty volume, or that one whose contents you do not want to remove. The main point is that you are not targeted for clean installation by your current startup amount. Such a clean installation is simple as you can do a clean install from the current startup drive, as the startup drive is not involved. No personalized startup environment is required. Only get the installer started and go.
The second choice is to make a simple install on the current startup drive, and probably the most popular one for the two. As the clean installation process eliminates the contents of the target disk, it is apparent that from the startup disk you can't boot and then attempt to delete it. The outcome would be a crashed Mac if it were possible. So if you plan to smooth El Capitan on your boot drive, it involves a further collection of steps to create a bootable USB flash drive with the El Capitan setup, remove the boot drive, then start the clean installation process.
You can download El Capitan old versions here.