watchOS 5.0 (build 16R5360a) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2018-08-24T17:14:04Z. The update package is 610 MB. This is a delta (partial) update that requires 4.3.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 9d1ee5fd0a0ee27156ec67c6859cacbd2f71c100.zip.