watchOS 5.3 (build 16U5566a) is an official Apple over-the-air update for the Apple Watch Series 4 (40mm, LTE), released on 2019-07-09T17:20:21Z. The update package is 115.67 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: 568246894c72497a707c63fe945d07117980e9ff.zip.