watchOS 5.3 (build 16U5566a) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm), released on 2019-07-09T17:19:55Z. The update package is 115.63 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: 96857e1d3faea391e0e012335f6c10c1955a1b2e.zip.