watchOS 5.1 (build 16R5589c) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-10-22T17:18:53Z. The update package is 732.15 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: b5e69082dbb9c953cfa6e1f48d640646b45885c0.zip.