watchOS 5.3 (build 16U5569a) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2019-07-16T17:14:28Z. The update package is 142.16 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: 5261f290b9a9a93f9c42fecd74749dc36bb265d8.zip.