Before downloading and installing Mac OS Sierra 10.12.6, ensure your Mac meets the minimum system requirements:
In the past, Apple made older OS versions difficult to find. The standard "Download" button in the App Store usually redirects to the newest compatible OS. However, Apple now maintains a hidden support section for legacy software. Mac Os Sierra 10.12.6 Dmg File Download
macOS Sierra 10.12.6 Combo Update (recommended to upgrade from any previous version of Sierra). Download macOS Sierra 10.12.6 Update - Apple Support Before downloading and installing Mac OS Sierra 10
Apple provides DMG files for several older macOS versions through its support website. Apple Support Download the DMG : Visit the Official Apple Support Download Page and select Sierra 10.12 Extract the Installer Open the downloaded InstallOS.dmg Double-click the InstallOS.pkg file within the disk image. Follow the prompts; this will place a new app called Install macOS Sierra Applications Run the Installer : Open your Applications folder and double-click Install macOS Sierra to begin the upgrade process. Apple Support Community Incremental Update (10.12.6) macOS Sierra 10
Prefer built-in methods: