watchOS 4.2.3 (build 15S600b) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2018-02-19T21:17:44Z. The update package is 161.63 MB. This is a delta (partial) update that requires 4.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 50c004090924668f70b3bdafddc0fa6f704539f9.zip.