watchOS 5.1 (build 16R591) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2018-10-30T17:21:12Z. The update package is 747.7 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: 4cd808c2fd092aea44270dc802ca9794cabaec0e.zip.