How do I update iOS 18 on my iPhone 11?

If your iPhone is actually running iOS 14 currently as shown in your device line. then the update may to be massive to be performed directly on the device, and would need to be done through a computer, Mac or PC.

If the update does not appear in Settings ➜ General ➜ Software Update or issues an error, see the following support document.

click here ➜ Update your iPhone, iPad, or iPod touch with iTunes - Apple Support

or here (on a recent Mac) ➜ Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

Update it from a Mac, or a Windows PC with iTunes or Devices installed. The gap between the current and latest versions is likely too large to be updated straight from the unit.

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An iPhone 11, as you show in your tagline, supports iOS 18.

What's preventing from simply choosing to install it in Settings > General > Software Update?