How to get iOS 18.0 on iPhone 17 Plus?

Sorry, but if your iPhone comes with iOS 26 preinstalled, you can not downgrade to iOS 18 anymore, it's the basic iOS version.

Even with an older iPhone running iOS 26, the downgrade to iOS 18 is not supported anymore by Apple.

There is no such thing as an iPhone 17 Plus.

It is not possible to run anything lower than iOS 26 on an iPhone 17, 17 Pro, 17 Pro Max, or iPhone Air.