how do I update my iPhone?

To update your iPhone, select Settings > General > Software Update. If your phone is eligible for an update, you will be prompted to do so. Check here for more information.

Update your iPhone or iPad - Apple Support

You were looking at an Apple document about updating to iOS 17.

If you have an iPhone 6s Plus....as indicated in your profile.....it will only update as far as iOS 15.8.1

If you want to update to iOS 16, you will need an iPhone 8.

If you want to update to iOS 17, you will need an iPhone XR, XS or higher model