watchOS 5.1.3 (build 16S5523a) is an official Apple over-the-air update for the Apple Watch Series 4 (44mm, LTE), released on 2018-12-10T18:10:42Z. The update package is 173.97 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: 10e36985b82f54021a009ca8706327a941dab25e.zip.