watchOS 5.1.3 (build 16S535) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2019-01-22T18:12:01Z. The update package is 798.19 MB. This is a delta (partial) update that requires 4.2.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 89c1c0c8281d9ea0aee46e1cc0dd5b84291a4789.zip.