watchOS 4.3 (build 15T5209a) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2018-03-12T20:07:42Z. The update package is 399.48 MB. This is a delta (partial) update that requires 4.0.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: c994e3247571c6bfbd29d0d0925456f6b26b18b7.zip.