watchOS 4.3 (build 15T5176c) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-02-07T18:09:44Z. The update package is 316.4 MB. This is a delta (partial) update that requires 4.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: ecc037eb4203ddd691c453a4df309cc6a6894318.zip.