watchOS 4.0 (build 15R372) is an official Apple over-the-air update for the Apple Watch (42mm), released on 2017-09-12T19:04:49Z. The update package is 582.01 MB. This is a delta (partial) update that requires 3.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 9e517482748731e6e6f818b41522e87f6837a1f3.zip.