tvOS 11.2.1 can be downloaded over the air through the Settings app on the Apple TV by going to System --> Software Update. Apple TV owners who have automatic software updates turned on will be upgraded to the tvOS 11.2.1 automatically.
As a minor 11.x.x update tvOS 11.2.1 focuses on performance improvements and bug fixes to address issues that were discovered following the release of tvOS 11.2. The update, released alongside iOS 11.2.1, likely re-enables remote access for shared users of the Home app.
Apple disabled remote access for shared users when implementing a server-side fix for a major HomeKit vulnerability last week. Apple at the time said remote access would be re-enabled this week through software updates.
Following today’s tvOS 11.2.1 update, the HomeKit vulnerability is fully patched and remote access for shared users has been reinstated.