Download an update for my iPhone 6s

Its telling you the reason right there under the "Download and Install" button.

"This update requires a Wi-Fi network connection to download"

Your iPhone is not currently connected to a Wi-fi network. Connect to Wi-Fi to download the update.

The file size is too large to download over an LTE connection. Even if you have unlimited data, iOS imposes download limits for updates over LTE to avoid data overage charges. Connect to Wi-Fi to download the update.