watchOS 5.3.1 (build 16U600) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2019-08-26T17:15:20Z. The update package is 1005.82 MB. This is a delta (partial) update that requires 3.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 1eb7da968610f7f300429352fd59208c031ab1a0.zip.