If your Mac is too new to download High Sierra from the App Store, you can use a utility called Mist (GitHub) .
hdiutil detach /Volumes/Install\ macOS\ High\ Sierra hdiutil convert /tmp/HighSierra.cdr.dmg -format UDTO -o /tmp/HighSierra.iso 4. Alternative Direct Download Links (Third-Party) download macos high sierra 1013 6 dmg file link
Click the Get button. This will open "Software Update" in System Settings. If your Mac is too new to download