watchOS 4.3.1 (build 15T5545b) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm), released on 2018-04-16T17:04:44Z. The update package is 333.64 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: 574064efb9cf56c83ca9bfc8df3450c52bf4c3b0.zip.