Important Note Before You Begin: macOS Sierra 10.12 was released in 2016 and is no longer supported by Apple. It does not receive security updates, and many modern apps (browsers, Slack, Zoom, etc.) no longer support it. Only proceed if you need it for legacy hardware or specific old software.
Visit Apple Support: Navigate to the official Apple "How to download macOS" page. download macos sierra 1012 dmg file install
Shift+Option+Command+R at startup) – this installs the original OS that Mac came with, which may be older than Sierra.Follow the onscreen prompts. When finished, you will find a new app called Install macOS Sierra in your Applications folder. 3. Create a Bootable USB Installer How to Download and Install macOS Sierra 10