watchOS 5.1.3 (build 16S5535a) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2019-01-07T18:16:31Z. The update package is 100.9 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: 81ccd2151f0e2bd00876542f1973edfabf8ea20b.zip.