watchOS 4.3 (build 15T5199f) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2018-03-06T18:08:20Z. The update package is 324.36 MB. This is a delta (partial) update that requires 4.2.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 3eb9e5c54cb9f32fbb39feaa39e24f6bbf1ff6b6.zip.