watchOS 5.1 (build 16R591) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2018-10-30T17:21:11Z. The update package is 961.93 MB. This is a delta (partial) update that requires 3.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: ccd26c4abe3043487b80f2687c200cbbce70d0a4.zip.