watchOS 5.3.1 (build 16U6600) is an official Apple over-the-air update for the Apple Watch Series 4 (40mm), released on 2019-08-26T17:16:44Z. The update package is 114.33 MB. This is a delta (partial) update that requires 5.2.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 50ee0e4efae2d0e78fe43a1b73c841465b22dc10.zip.