1. First of all, you need to remove Windows 8 from boot loader. For this you can download software from this link http://www.softpedia.com/get/System/OS-Enhancements/EasyBCD.shtml. Install it completely. Once installed, execute the program.
2. Run the program and click Edit Boot Menu from the left sidebar
3. Select Windows 8 Preview and click Delete. Now check the previous version of windows and click Save Settings.
4. Now click on BCD Backup/Repair on the left side and choose Recreate/repair boot files. Then click Perform Action. This action will delete windows 8 from boot loader. If by any chance, you are seeing Windows 8 again, repeat the steps mentioned above.
5. Now you need to right click the drive on which Windows 8 is installed and simply format it. That is all you need to do in order to downgrade from windows 8 to windows 7