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 555.73 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: f4e6d1fafb9328d4d825baab1e66b53151f56540.zip.