watchOS 4.0 (build 15R5371a) is an official Apple over-the-air update for the Apple Watch (42mm), released on 2017-08-28T16:53:21Z. The update package is 29.08 MB. This is a delta (partial) update that requires 4.0 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 9a70012978bbc64c753bea36151a144aaec0e95e.zip.