watchOS 5.3.1 (build 16U6600) is an official Apple over-the-air update for the Apple Watch Series 4 (44mm, LTE), released on 2019-08-26T17:17:16Z. The update package is 114.42 MB. This is a delta (partial) update that requires 5.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 7f20e51841ce3c33cde3b72d27796b7018ee809d.zip.