watchOS 4.3 (build 15T5199f) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2018-03-06T18:08:21Z. The update package is 135.33 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: e8640ade4d71944fb353dab4bd93fc5e401d8ec7.zip.