watchOS 4.0 (build 15R372) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2017-09-12T19:05:11Z. The update package is 539.13 MB. This is a delta (partial) update that requires 3.2.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 712a40f1ac1529df727800d876c3f823b2e0a07a.zip.