watchOS 5.1 (build 16R5559d) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm), released on 2018-09-18T17:23:15Z. The update package is 383 MB. This is a delta (partial) update that requires 5.0 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 9c37d9894bb1a87b398e8ba73472ec31b20b7715.zip.