watchOS 5.3.9 (build 16U693) is an official Apple over-the-air update for the Apple Watch Series 4 (40mm, LTE), released on 2021-01-26T18:27:01Z. The update package is 474.67 MB. This is a delta (partial) update that requires 5.1.2 already installed. This update is currently signed by Apple, so it can be applied from Settings › General › Software Update.

Payload file: ea3ff79d100a2c130314b932e482a06cbdcadea8.zip.