watchOS 4.0 (build 15R5365a) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2017-08-22T16:58:49Z. The update package is 586.75 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: 670a9d108cc94b80494c8cd649b81ac5b7432bf2.zip.