watchOS 5.1.3 (build 16S535) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2019-01-22T18:11:39Z. The update package is 148.01 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: 3c6d2183f4834cacf3a0cb470c29694680d08f88.zip.