watchOS 5.1.3 (build 16S5535a) is an official Apple over-the-air update for the Apple Watch Series 4 (44mm), released on 2019-01-07T18:16:48Z. The update package is 186 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: 93f9d25ec1df13daee1a34165f104e3cfe445948.zip.