watchOS 5.3.1 (build 16U600) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2019-08-26T17:14:46Z. The update package is 831.19 MB. This is a delta (partial) update that requires 4.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: a108f3a35a2992f66c8d52ef03bb1e47937a760c.zip.