Compatible iPhones for iOS 18

Compatibile devices are listed at the bottom of the article linked below.

As to how to download, that answer remains the same s the last time you asked, unless of course, you are a developer, or wish to brick your phone.

iOS 18 Preview - Apple

And per usual: don’t run a beta on any iPhone you need to work, with any apps you need to work, or with any iCloud data you need to be available and unconverted and uncorrupted.

Do have a backup of your iPhone from before loading the beta, as restoring that backup the only way to exit the beta prior to the release of iOS 18 later this year.