watchOS 5.1 (build 16R5559d) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2018-09-18T17:23:00Z. The update package is 773.1 MB. This is a delta (partial) update that requires 4.3.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 5ea44400802b08bc3386642d5f41367b9fb92734.zip.