tvOS 8.4.2 (build 12H606) is an official Apple over-the-air update for the Apple TV 3 (2013), released on 2016-12-12T19:33:11Z. The update package is 487.15 MB. This is a delta (partial) update that requires 7.1.1 already installed. This update is currently signed by Apple, so it can be applied from Settings › General › Software Update.

Payload file: 4aa7ae003ac03fc06c71b9634d6bad9152790024.zip.