watchOS 4.0 (build 15R5365a) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2017-08-22T16:58:49Z. The update package is 140.65 MB. This is a delta (partial) update that requires 4.0 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 3775106f412d1f4afd28f0311c0ff1e02c171719.zip.