watchOS 5.1.3 (build 16S5523a) is an official Apple over-the-air update for the Apple Watch Series 4 (44mm, LTE), released on 2018-12-10T18:10:43Z. The update package is 164.95 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: 425f7e5b7260e3ab0a6e02c97046c5575c641625.zip.