watchOS 5.1.3 (build 16S5535a) is an official Apple over-the-air update for the Apple Watch Series 4 (44mm), released on 2019-01-07T18:16:50Z. The update package is 76.65 MB. This is a delta (partial) update that requires 5.1.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 33496705ea77790d715dddc1ec960fe3773dd080.zip.