Follow these steps on how to reformat your Windows 8:
Step 1. Insert your Windows 8 OS.
Step 2. Go to Settings > Change PC settings.
Step 3. Under the PC settings, click on "General".
Step 4. Scroll down your mouse, and find "Remove everything and reinstall Windows".
Step 5. Click on "Get Started".
Step 6. Read the message, and click "Next".
Step 7. You need to choose which type you want to perform. If you want to only remove your files, or clean the drive fully.
Step 8. You will be asked again if you really want to format your computer. You just need to confirm and wait.