watchOS 5.3.1 (build 16U600) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2019-08-26T17:14:41Z. The update package is 983.64 MB. This is a delta (partial) update that requires 3.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 7391dc49bf1f320c8c7640b0cffa65a73aeb38aa.zip.