watchOS 5.3.1 (build 16U600) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2019-08-26T17:14:48Z. The update package is 831.15 MB. This is a delta (partial) update that requires 4.2.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 5f431be5ae949b54cd76d70b10580d67afcb9d12.zip.