How can I update my iPhone X to iOS 18.4?

Your iPhone X can only be updated to iOS 16.7.11. It has stopped receiving the latest iOS software updates due to hardware constraints and as a result cannot be updated to iOS 18. The iPhone XR and later can do so.

You cannot update your iPhone X to iOS 17 or iOS 18.

For the iPhone X, iOS 16 is the end of the line.