tvOS 11.2.1 (build 15K152) is an official Apple over-the-air update for the Apple TV 4 (2015), released on 2017-12-13T18:31:01Z. The update package is 694.26 MB. This is a delta (partial) update that requires 10.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: d163115fd24f92a332b72b1940f329745ade046a.zip.