watchOS 5.1.3 (build 16S5523a) is an official Apple over-the-air update for the Apple Watch Series 4 (44mm), released on 2018-12-10T18:10:36Z. The update package is 149 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: 9d0653c8429b0c5079feb249f9f549c87fc4d38e.zip.