watchOS 4.0 (build 15R5371a) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2017-08-28T16:53:39Z. The update package is 529.64 MB. This is a delta (partial) update that requires 3.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 1488834ae5df98c64e971af1e57169360f94f3fc.zip.