watchOS 4.0 (build 15R5371a) is an official Apple over-the-air update for the Apple Watch Series 2 (42mm), released on 2017-08-28T16:53:26Z. The update package is 587.74 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: 2849c58c260c340c6121835e116412f0d3425554.zip.