watchOS 5.2.1 (build 16U5079d) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2019-03-28T17:11:04Z. The update package is 441.9 MB. This is a delta (partial) update that requires 5.1.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 9f818f756b770601e72ff7e7d4a7be42790d13da.zip.