So my MacBook Air under "About this Mac" is running OS X Mavericks 10.9.5, but under "Software Update..." thinks that macOS Sierra 10.12.1 is installed. I've tried rebooting to see if the system would update to Sierra, but it just stays at 10.9.5. App Store has no option for updating because it thinks Sierra already is installed. And my MacBook Air is Late 2010 so should be compatible with Sierra. Not sure what to do!