Today, Apple released what is probably going to be the final update to macOS Sierra version 10.12.6. This update is a free update to all users currently running macOS Sierra. There are no major features or changes with this update, it simply focuses on security, stability and compatibility for your Mac.
To perform this update simple go to the Apple menu in the top-left of you Mac and then click App Store. There is an "Updates" section of the App Store, that is where you will find the update.
This will most likely be the last incremental update for macOS Sierra as Apple begins to focus more energy into the next iteration of it's macOS, High Sierra, coming sometime in the fall.