iOS12 beta crashed doing update
Put it into DFU mode and use iTunes to restore it. You can restore it to either iOS 11 or iOS 12 beta 9.

Here are instructions on entering DFU mode: https://www.theiphonewiki.com/wiki/DFU_Mode (Since the instructions vary between devices and you did not mention which device you have, I won't copy and paste them all to this page)

When it's connected to iTunes you should be able to press Restore. If that doesn't work, try restoring a specific IPSW file. You can download files here: https://ipsw.me

You can use the IPSW by holding option (mac) or shift (windows) when pressing the "Update" button. It will prompt you to select the IPSW.