watchOS 4.0 (build 15R5365a) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2017-08-22T16:58:54Z. The update package is 541.95 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: d930d41534bba72e1216fb7014221b853928250a.zip.