watchOS 4.1 (build 15R5843a) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2017-10-16T17:06:01Z. The update package is 326.81 MB. This is a delta (partial) update that requires 4.0 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: adfa529daca0249a64b73b167e003e0fee5197c2.zip.