watchOS 4.0 (build 15R5365a) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2017-08-22T16:58:42Z. The update package is 573.01 MB. This is a delta (partial) update that requires 3.1.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 386fb9b6bfd3638d1a518841f3e65e29f57237c1.zip.