watchOS 5.3.1 (build 16U600) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2019-08-26T17:15:22Z. The update package is 1003.4 MB. This is a delta (partial) update that requires 3.1.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: d17fc3ec2cc7e47f618556ad1e46d34cf3ef190f.zip.