I had installed FreeBSD but can't boot, it only boots GhostBSD. I had installed GhostBSD to the disk before. How to fix that? thanks

You'll have to give more details. Where does it fail exactly?

Boot using the usb thumb drive with GhostBSD ISO installed.
Once you see the desktop, open a terminal and type the following.

su
gpart destroy -F ada0

Once done, remove the usb thumb drive with GhostBSD and shutdown the computer.

You should be able to install another OS without a problem now.

I had run that command, but it still boots ghostbsd. It finally works after I format it under linux.