Ten days after you upgrade to Windows 10, your previous version of Windows will be automatically deleted from your PC. However, if you need to free up disk space, and you’re confident that your files and settings are where you want them to be in Windows 10, you can safely delete it yourself. Keep in mind that you'll be deleting your Windows.old folder, which contains files that give you the option to go back to your previous version of Windows. Deleting your previous version of Windows can’t be undone.
In the search box on the taskbar, type settings, then choose it from the list of results.
Select System > Storage > This PC and then scroll down the list and choose Temporary files.
Under Remove temporary files, select the Previous version of Windows check box and then choose Remove files.