watchOS 5.1.3 (build 16S5535a) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2019-01-07T18:16:25Z. The update package is 135.87 MB. This is a delta (partial) update that requires 5.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: d104b62ad1b003f6eda4fd8affb835f8b93aee09.zip.