watchOS 4.3 (build 15T5212a) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2018-03-16T20:09:41Z. The update package is 360.73 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: 35b78a329d85b74e7f4909ff9f77f20ce78264d1.zip.