watchOS 4.0 (build 15R5371a) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2017-08-28T16:53:48Z. The update package is 541.76 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: ed37eb9f031e91a5315c8ec98d711ef447d2e2cb.zip.