watchOS 5.1.3 (build 16S5535a) is an official Apple over-the-air update for the Apple Watch Series 4 (44mm, LTE), released on 2019-01-07T18:16:59Z. The update package is 164.38 MB. This is a delta (partial) update that requires 5.1.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 0cda0bca2d11cb91f41fe193f49afd345cadf938.zip.