watchOS 5.1.3 (build 16S535) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2019-01-22T18:11:59Z. The update package is 821.37 MB. This is a delta (partial) update that requires 4.0.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 1f4a83463cf34053a575cf83a57c9f35094fde67.zip.