How do I update an IPhone 6s?

Navigate to:

Settings -> General -> Software Update

You should be able to update from that panel. Make sure you're connected to WiFi and your battery is at 100% OR your iPhone is plugged into a charger.

You install the update when you’re on a good Wi-Fi signal with a well charged battery.

lise150 wrote:

is red??

Please describe further. There is likely something you can tap - such as an exclamation point.