While the OS itself is free from Apple, third-party sites advertising "Free Downloads" pose significant security risks: Download macOS Sierra 10.12.5 Update for 2017 iMacs
Apple provides the software for free to users with compatible hardware. You can obtain it safely through: advertisement macOS Sierra 10.12.5 Free Download
Standalone installers are available, such as the macOS Sierra 10.12.5 Combo Update , which allows updating from any prior version of Sierra. While the OS itself is free from Apple,
The most direct method; check the "Updates" tab for the built-in Software Update function. advertisement macOS Sierra 10.12.5 Free Download