watchOS 4.0 (build 15R5371a) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2017-08-28T16:53:50Z. The update package is 204.18 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: 3bfa463cd79609df6c784b505c8325643185f03a.zip.