The iOS 11.2.1 firmware is available over-the-air in the Settings app. Browse for General -> Software Update and tap the Download and Install button to adopt this latest iOS version. The update is available to all devices that are compatible with iOS 11.
iOS 11.2.1 HomeKit Remote Sharing Fix
A major HomeKit security bug surfaced last week. It allowed unauthorized access to HomeKit compatible accessories like smart locks, light bulbs and more. Apple fixed it immediately server-side, however this quick repair required disabling of the remote access for shared users. Now, the bug has been properly fixed and remote access has been restored to all users that update to iOS 11.2.1.
Fact: iOS 11.2.1 comes with build number 15C153.