Apple on Tuesday released macOS Sierra for its laptop and desktop computers. The software update is free, and compatible with iMacs and MacBooks from 2009 or later, as well as MacBook Airs, MacBook Pros, Mac Minis and Mac Pros from 2010 or later.
If you own any of the above hardware, you can update to macOS Sierra by navigating to the Mac App Store and locating the macOS Sierra page (or you can just search the App Store for macOS Sierra). Click on 'Download' and follow the on-screen instructions.