If you upgraded to Windows 8.1 from Windows 8.1 Preview or to Windows RT 8.1 from Windows RT 8.1 Preview, some files used by your previous version of Windows will be left in the Windows.old folder. These files will be automatically removed within 28 days of upgrading, or you can use Disk Cleanup to remove them yourself.
Before you use Disk Cleanup, make sure that all of your files and settings are where you expect them to be. Deleting the Windows.old folder can’t be undone.
Open Control Panel by swiping in from the right edge of the screen, tapping Search (or if you're using a mouse, pointing to the upper-right corner of the screen, moving the mouse pointer down, and then clicking Search), entering Control Panel in the search box, and then tapping or clicking Control Panel.
In the Control Panel search box, type Admin, tap or click Administrative Tools, and then double-tap or double-click Disk Cleanup.
If you're prompted to choose a drive, select the drive that you just installed Windows on, and then tap or click OK.
In the Disk Cleanup dialog, tap or click Clean up system files.
If you're again prompted to choose a drive, select the drive you just installed Windows on, and then tap or click OK.
Select the Previous Windows installation(s) check box, and any other check boxes for the files you want to delete, and then tap or click OK.
In the message that appears, tap or click Delete files.