watchOS 5.3.1 (build 16U6600) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2019-08-26T17:15:40Z. The update package is 138.16 MB. This is a delta (partial) update that requires 5.2.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: e3614d292a07d1d26072692926414a996c772bad.zip.