watchOS 5.3 (build 16U5537b) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2019-05-28T17:12:32Z. The update package is 498.42 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: 8fc390175eb2460a03c9686c89c12b1df9430375.zip.