watchOS 5.3 (build 16U5569a) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2019-07-16T17:14:30Z. The update package is 499.46 MB. This is a delta (partial) update that requires 5.1.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: dd6ff55d20b5216aa5ed97b33acab486852d3792.zip.