If you are already running an earlier version of Sierra (like 10.12.5), you can download the standalone 10.12.6 Update
Apple provides a direct download link for the macOS Sierra installer in DMG format: Mac Os Sierra 10.12.6 Dmg File Download
: If you already have an earlier version of Sierra, you can download the macOS Sierra 10.12.6 Combo Update If you are already running an earlier version
Before downloading, ensure your Mac is compatible. macOS Sierra requires at least 2GB of RAM 8.8GB of available storage Supported Models: : Late 2009 or newer. MacBook Pro : Mid 2010 or newer. MacBook Air : Late 2010 or newer. : Mid 2010 or newer. : Late 2009 or newer. : Mid 2010 or newer. 3. Creating a Bootable USB Installer Mac Os Sierra 10.12.6 Dmg File Download