watchOS 5.2.1 (build 16U5079d) is an official Apple over-the-air update for the Apple Watch Series 4 (44mm, LTE), released on 2019-03-28T17:11:44Z. The update package is 460.96 MB. This is a delta (partial) update that requires 5.1.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 4fc967d65b5ab17bc453b5a8cf642ef83d7cf645.zip.