The macOS operating system is one of the most popular computer systems developed by Apple Inc. since 2001. According to Wikipedia, it is the second most used operating system after Windows. If you currently use an older macOS version, you can upgrade your old operating system to High Sierra for free.
How to Download macOS High Sierra 10.13 using Apple Store
MacOS High Sierra 10.13 includes all the new APFS file systems, improved graphics support, Safari 11, updates in the Photos application, fresh features, other adjustments, changes, and various advanced security/privacy features.
Download macOS High Sierra 10.13
You can only download the Apple Mac OS X update package from the Apple Store:
https://itunes.apple.com/us/app/macos-high-sierra/id1246284741?mt=12
After downloading the new macOS update from the Apple Store, you can upgrade your system with just a few clicks.