watchOS 5.1 (build 16R591) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-10-30T17:21:03Z. The update package is 965.72 MB. This is a delta (partial) update that requires 3.1.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 6ac22d204625050e744e8a7799f38914c4142774.zip.