watchOS 5.1.3 (build 16S5531a) is an official Apple over-the-air update for the Apple Watch Series 4 (44mm, LTE), released on 2018-12-19T21:06:42Z. The update package is 91.56 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: 5365206768772ad6e3feab70b034911e5e71eb1d.zip.