watchOS 5.1 (build 16R5559d) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2018-09-18T17:22:57Z. The update package is 412.78 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: 933e21b473a44d02cfa54262a9a6eb60977b52c6.zip.