watchOS 4.3 (build 15T5187e) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2018-02-21T18:12:09Z. The update package is 371.4 MB. This is a delta (partial) update that requires 4.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: ec8e6c60c85383cda0322ccfcc8cf538fcedd997.zip.