MacOS/Upgradeについて、ここに記述してください。
1. Appleの案内
https://www.apple.com/macos/how-to-upgrade/
Chances are, your Mac can run macOS High Sierra.
Mac computers introduced in late 2009 or later can be upgraded to macOS High Sierra.
Then, if your Mac is running OS X Mountain Lion 10.8 or later, you can upgrade directly to macOS High Sierra.
1.1. How do I upgrade from Lion version 10.7.5 to Yosemite or later
10.11 (El Capitan) に更新せよということらしい。(freeとある）
2. 別の案内
https://macpaw.com/how-to/how-to-upgrade-your-mac-to-macos-sierra