watchOS 4.0 (build 15R372) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2017-09-12T19:05:12Z. The update package is 598.18 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: f36337628ea4ad66e5122b563fe4601bc759b6e8.zip.