Operating System Version Numbers, February 2017.
Across the past two months, Apple has released system updates for all devices:
How To Check Your OS Version and Perform an Update
Click the at the top left of the Finder and select About This Mac. Click the “Software Update” button.
Open Settings and select General and then “Software Update.”
Open Settings and select General and “About.” To check for an update, open Settings, then System and choose “Software Updates.”
On your iPhone, open the Watch app and select General and “Software Update.”