Cleanup Your Storage
If you're running out of space on your iPhone or iPad, you might need to do some cleanup in order to install the iOS 12 beta.
The iOS 12 beta requires a sizable chunk of space on your internal storage. If you've got less than 1GB available, you won't be able to install it.
If you're on the threshold, use this time to delete files you no longer need. Maybe that's videos you no longer want or music you no longer listen to.
Cleaning up your hard drive could also improve your device's overall speed.