How do I get the new update

iOS 18 is not out yet.

When it is, it will be available for those phones shown on the compatibility list on this page

https://www.apple.com/ios/ios-18-preview/

or for newer phones that aren't out yet.

What is your current iOS version and what version are you looking to update to?

What iPhone do you have?

Are you asking about the iOS 18 update? That will not be available for another month or two.

If not, which update were you looking for?