watchOS 5.1 (build 16R5584a) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-10-15T17:09:28Z. The update package is 732.14 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: c47676f3ed54e395fc0d4a1a8ff072f00cc37506.zip.