watchOS 4.0 (build 15R5371a) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2017-08-28T16:53:40Z. The update package is 529.24 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: ba71dd20336f401a59ef21cc02f5ebab9f163a31.zip.