watchOS 4.0 (build 15R5372) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2017-09-12T19:05:34Z. The update package is 88.18 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: b5793dcdd27542eff9dbb7873580d55fc15039e0.zip.