If you are already running an older version of Sierra (like 10.12.5) and just need to update to 10.12.6, you can download the standalone update packages:
Where to get the installer safely
The flash drive will be formatted, so back up any existing data. Formatting the USB Drive Plug the USB drive into your Mac. Open (Finder > Applications > Utilities). Select your USB drive from the left sidebar. Click Erase at the top. 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
Restores snappy performance to vintage aluminum iMacs and older unibody MacBook Pros that lag under heavier, modern operating systems. If you are already running an older version
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If you are running an older macOS version that still supports App Store legacy redirects, you can fetch the installer through the app platform. Open the App Store on your compatible Mac.
Apple hosts a direct link to the full Sierra 10.12.6 installer on its content delivery network. This is the safest method, as it comes straight from Apple’s servers. Select your USB drive from the left sidebar
Guide: Downloading macOS Sierra 10.12.6 DMG If you need to download the macOS Sierra 10.12.6 DMG file, it is essential to use official Apple sources to ensure the security and stability of your system. Although Sierra is an older operating system, Apple still provides access for users with compatible hardware. Official Download Options
Sierra enabled online shopping with Apple Pay directly in Safari. Checkout became a one-click process, with payment authenticated via Touch ID on an iPhone or Apple Watch.