watchOS 5.3.1 (build 16U600) is an official Apple over-the-air update for the Apple Watch Series 4 (44mm, LTE), released on 2019-08-26T17:17:10Z. The update package is 494.55 MB. This is a delta (partial) update that requires 5.1.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 12995badf9f85ca5548c7addb2911213b70c285b.zip.