The iOS 12 beta is out now and you can download it by installing the iOS 12 beta profile on your iPhone or iPad.
The developer beta is officially only available to download to those who have a developer account with Apple. However, you can get the iOS 12 update unofficially as well on your iPhone.
If you’ve already tried downloading iOS 12 Beta with a developer configuration profile, but met with the “Software Update Failed. An error occurred downloading iOS 12 Developer Beta” error. Then try updating to iOS 12 using IPSW firmware file.
You can get iOS 12 IPSW firmware files for all iOS 12 compatible devices from the link below. The firmware files can be installed without a developer account to your compatible iOS devices.