iOS 9.2.1 also fixes a problem “that could prevent the completion of app installation when using an MDM server.”
Before installing the updates, back up the data on your Mac and iOS device.
To install the iOS update on your device through a Wi-Fi or mobile connection:
The iOS 9.2.1 update is 41MB. Your device will need to restart once to complete the installation.
You can also update your iOS device by connecting it to your Mac and running iTunes.
To install the OS X update on your Mac:
Your Mac will need to restart to complete the update.
These updates put the devices one step closer to the major changes that will be introduced in iOS 9.3 and OS X 10.11.4. Learn more about the new features in iOS 9.3, which include Night Shift, new features in Notes, and new features geared towards education. OS X 10.11.4 includes support for Live Photos, other enhancements.