watchOS 5.1 (build 16R5559d) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2018-09-18T17:22:53Z. The update package is 772.15 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: 93d1269c173f3305d8d783d5d20989160461c176.zip.