watchOS 5.1.3 (build 16S6535) is an official Apple over-the-air update for the Apple Watch Series 4 (44mm, LTE), released on 2019-01-22T18:14:09Z. The update package is 99.19 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: 52fd35cc68c7b8bc6d0e7781fbd15b9e9387196b.zip.