watchOS 5.3.1 (build 16U600) is an official Apple over-the-air update for the Apple Watch Series 4 (40mm, LTE), released on 2019-08-26T17:16:58Z. The update package is 474.58 MB. This is a delta (partial) update that requires 5.1.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 22800698cb3f4edb24e6cfb152821e0607d6a3fb.zip.