watchOS 4.2.3 (build 15S600b) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-02-19T21:17:35Z. The update package is 558.96 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: d95d799040f3801fb2bab8ff4f292c64e10146bc.zip.