watchOS 5.1.3 (build 16S5523a) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2018-12-10T18:10:14Z. The update package is 188.04 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: 26e107a69d844b43c8665c28f3a4e71190bb3b60.zip.