1. Open up the Charms Search Bar on the right hand side of Windows 8:
2. Type cleanmgr in the Search Bar and press Enter:
3. When Disk CleanUp appears, check Previous Windows Installation and click OK. It would take some time for Windows 8 to clean up and delete your previous Windows Installation.
Make sure that you backup all of the important files, folders and documentations in order to permanently delete your previous Windows Installations. Once removed, you wouldn’t be able to restore or recover the data.