watchOS 4.0 (build 15R5371a) is an official Apple over-the-air update for the Apple Watch (42mm), released on 2017-08-28T16:53:17Z. The update package is 568.81 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: 73dd17f5fc04b25ea2bb9c10e48a5215e2565559.zip.