watchOS 5.3.1 (build 16U600) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2019-08-26T17:15:39Z. The update package is 136.44 MB. This is a delta (partial) update that requires 5.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: bd15d513a126bd019ac2faa4c6a26663dfa1d630.zip.