watchOS 4.3.1 (build 15T5545b) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2018-04-16T17:04:30Z. The update package is 181.12 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: 1d813020f49b3a8ce234ff5471cf80ce9eff3220.zip.