watchOS 5.3.1 (build 16U600) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2019-08-26T17:15:30Z. The update package is 484.02 MB. This is a delta (partial) update that requires 5.0.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: edb04f21b063e56f413818a19cd39a1cadd7674b.zip.