watchOS 5.3 (build 16U5545c) is an official Apple over-the-air update for the Apple Watch Series 4 (44mm, LTE), released on 2019-06-11T17:14:19Z. The update package is 156.95 MB. This is a delta (partial) update that requires 5.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 183acee3344cf0f3f3aeb2bdcf256745b0767ebd.zip.