watchOS 5.1.3 (build 16S535) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2019-01-22T18:11:15Z. The update package is 773.24 MB. This is a delta (partial) update that requires 4.0 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 2b846a8e3e45f27d13cac317728ceecfa5fc8dfa.zip.