watchOS 5.3 (build 16U5558b) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2019-06-24T17:25:07Z. The update package is 168.32 MB. This is a delta (partial) update that requires 5.2.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 7f0f54037896870091afcddb85998552d1cf7a33.zip.