https://updates.cdn-apple.com/2019/cert/091-123123-20190910-XXXXX/InstallOS.dmg
If you specifically need a (Disk Image) rather than the .app installer—for example, for virtualization software—you can extract it from the official download: Mac Os Sierra 10.12.6 Dmg File Download
: The DMG file is used by administrators to create bootable USB installers using Terminal commands like createinstallmedia , which is essential for refurbishing older machines. Apple Support Community Why This Specific Version Matters End-of-Life Landmark https://updates
Run that app from your Applications folder to begin the upgrade. 2. macOS Sierra 10.12.6 Updates Mac Os Sierra 10.12.6 Dmg File Download