iOS 11.4 (build 15F5071a) is an official Apple over-the-air update for the iPhone 6s, released on 2018-05-07T17:11:49Z. The update package is 225.83 MB. This is a delta (partial) update that requires 11.4 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 8cb1d1f4d371a6d6c39b62c7751443220cc011c4.zip.