watchOS 4.3 (build 15T5199f) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2018-03-07T02:04:20Z. The update package is 360.06 MB. This is a delta (partial) update that requires 4.2.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 4e057bfea744bc9f5c3b9d9ab36ddbb18c2b2a65.zip.