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 529.26 MB. This is a delta (partial) update that requires 3.2.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 515dfd205c2babb1e13cac5cf9a61fa7f2d674bf.zip.