watchOS 5.1.3 (build 16S535) is an official Apple over-the-air update for the Apple Watch Series 4 (44mm, LTE), released on 2019-01-22T18:14:03Z. The update package is 166.13 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: b8f72a148d1c7de762be4e2f6672e7791b728590.zip.