watchOS 5.3 (build 16U5569a) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2019-07-16T17:14:33Z. The update package is 142.1 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: a1be7ea2eb0cb3a9ef0173db01f07c81e7dbc6ac.zip.