I update my iPhone X to iOS 12 by doing this...
Step 1: https://beta.apple.com/sp/betaprogram/
Step 2: Enroll your Device
Step 3: Settings to Software Update
Step 4: iOS 12 Golden Master
Step 5: Don't have to wait five days for iOS 12
...
https://www.youtube.com/watch?v=McWwEOfh_VA