If you are running an older system software and want to upgrade, but you aren’t ready to go all the way to 10.13 High Sierra just yet, Apple still does offer 10.12 Sierra for download, though it’s hidden on the App Store.
You can download it here.
For most people I recommend going to High Sierra. It has been out for quite some time and most of the major bugs have been resolved. But some people have compatibility concerns. If you do get stuck using Sierra you will continue to receive security patches until September 2019.