watchOS 4.3 (build 15T5209a) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2018-03-12T20:07:38Z. The update package is 360.61 MB. This is a delta (partial) update that requires 4.2.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 8548b5d829b7ff0e2efc4d30fd8714ed6fa1ef52.zip.