watchOS 4.3.1 (build 15T5535b) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2018-04-02T17:13:30Z. The update package is 117.95 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: 62111c04b56f7e355fef6bf3c492f4bb622d393d.zip.