How can I update my iPhone to iOS 18?

See how here:

Update your iPhone or iPad - Apple Support

Note than an iPhone Xs/Xr/SE2 (or newer) is required to run iOS18.

If your iPhone is on this list, you can update it to the latest release of iOS 18.

iPhone models compatible with iOS 18 - Apple Support

If it isn’t, you can’t.

What does it say at settings - general - software update? Do you have a phone that can update to iOS 18?

You never mentioned which model iPhone you have.

If your iPhone is too old, it has reached the end-of-life support for Software Updates to the latest iOS operating system, depriving you of new features created many years ago.

Many modern apps are not compatible, limiting the usability of your device.

You are invited to visit an Apple retail store to discuss your iPhone upgrade options.

Learn about the ⇢ iPhone Upgrade Program. ⇠📲

If your iPhone is too old for a trade-in, then the only option is to recycle it for free. ♻️

Good luck! 👋🏼😉