watchOS 4.3 (build 15T5212a) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2018-03-16T20:09:42Z. The update package is 158.51 MB. This is a delta (partial) update that requires 4.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: dc033a737f2711a6ff467966d1a2ff99fe13c177.zip.