It will depend if you want to preserve the root volume or not.
If you do, you will need a second volume (doesn't matter which type). Move the root volume data over to it. Then make it the root volume, by changing the volume option 'root' to on, then reboot. Now you can destroy the old volume and remake it into an aggregate.
If you want to just totally start over you can boot to maint. mode by booting the box then pressing CTRL-C when it prompts you. Then select option 4a. Keep in mind you will need to re-setup everything including all license keys.
Hope this helps and feel free to post follow-up questions.
-- Adam Fox