Do you want to wipe your data, or just do a clean installation? If you want to wipe your data, go to the Storage screen, select your pool, and click Detach Pool (button at the bottom with the red X). Check the box for "mark disks as new". Disks will then be wiped.
Then install FreeNAS to a new USB stick, boot from that stick, and you're in business with a clean installation of the latest stable release and blank hard drives. If you don't want to wipe your data, omit the step above of checking the "mark disks as new".