watchOS 5.3 (build 16U5566a) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2019-07-09T17:19:44Z. The update package is 171.35 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: c03bd840ab6f95b0a9f8a1e10cf1eca9def3ebee.zip.