watchOS 5.3.9 (build 16U693) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2021-01-26T18:26:17Z. The update package is 532.42 MB. This is a delta (partial) update that requires 5.0 already installed. This update is currently signed by Apple, so it can be applied from Settings › General › Software Update.

Payload file: bf28322aa192e61ba0efbdd60412320460043f70.zip.