watchOS 5.1.3 (build 16S5535a) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2019-01-07T18:16:24Z. The update package is 735.76 MB. This is a delta (partial) update that requires 4.3.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 2c0116981f10b1ef26aacb47bc487c17e3ea2935.zip.