How to upgrade my Iphone 8 to IOS 18?

Two things to note. An iPhone 8 cannot be upgraded beyond iOS 16.7.8.

Also, iOS 18 has not been released yet. It is still in beta testing. A Beta version on iOS 18 can be installed on newer phones but not on an iPhone 8.

I heard from the blue shirt wearing Apple guy in the 'Merican mall that I can upgrade my Iphone 8 from IOS 16 to IOS 18. Is that true?

No, it is not true. The iPhone 8 cannot even update to iOS 17, much less iOS 18.

If you want to run iOS 18, you will need an iPhone XR or higher model.