watchOS 4.3 (build 15T5187e) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2018-02-21T18:12:13Z. The update package is 411 MB. This is a delta (partial) update that requires 4.0 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: e2369a0313089795866bf9abd4331e8ce98e7dcd.zip.