watchOS 5.1 (build 16R5577c) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2018-10-09T17:11:58Z. The update package is 409.1 MB. This is a delta (partial) update that requires 5.0 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: e5cacdf3298ed65bb060ad691591f50eba492687.zip.