watchOS 5.3 (build 16U5537b) is an official Apple over-the-air update for the Apple Watch Series 4 (44mm, LTE), released on 2019-05-28T17:13:17Z. The update package is 149.75 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: df892221b17effc7139747473024c97365b7b4a6.zip.