watchOS 5.3.9 (build 16U693) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2021-01-26T18:26:24Z. The update package is 515.96 MB. This is a delta (partial) update that requires 5.1 already installed. This update is currently signed by Apple, so it can be applied from Settings › General › Software Update.

Payload file: e64fcd8133598e238167c31295b1209b57745fc8.zip.