watchOS 5.1 (build 16R5559d) is an official Apple over-the-air update for the Apple Watch Series 2 (42mm), released on 2018-09-18T17:22:31Z. The update package is 727.95 MB. This is a delta (partial) update that requires 4.3.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 387804041c420e173e36e6ea59fe2ca8f1251614.zip.