watchOS 5.3.1 (build 16U600) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2019-08-26T17:16:03Z. The update package is 883.69 MB. This is a delta (partial) update that requires 4.2.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 74c7555ed13b8b54b47a06e449927dc1f0432a11.zip.