watchOS 5.1.3 (build 16S535) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2019-01-22T18:11:47Z. The update package is 797.32 MB. This is a delta (partial) update that requires 4.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 4822c880a57c1de06a01ab3caec697bfa117a914.zip.