Updating iPhone 15 Plus from iOS 17 to iOS 18 instead of iOS 26

If I have an iPhone 15 Plus that is out of the box on iOS 17 and update it to iOS 18 instead of iOS 26? Is it possible now? Or will it be possible only on iOS 26?

[Re-Titled by Moderator]

Original Title: Can I have an iPhone 15 Plus out of the box on iOS 17 and update it to iOS 18 instead of iOS 26?