首先，微软官方给出的正确姿势是这样的（双语）：

在升级到 Windows 10 十天后，以前版本的 Windows 将自动从电脑中删除。 不过，如果你需要释放磁盘空间，并且你确信文件和设置位于你希望它们在 Windows 10 中所处的位置，你可以自行将其安全删除。 请记住，你将删除 Windows.old 文件夹，该文件夹包含的文件可供你选择回退到以前版本的 Windows。 删除以前版本的 Windows 这一操作不可撤销。

在任务栏上的搜索框中，键入“设置”，然后从结果列表中选择它。
选择“系统”>“存储”>“这台电脑”，然后滚动到列表底部，选择“临时文件”。
在“删除临时文件”下，选择“以前版本的 Windows”复选框，然后单击“删除文件”。

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Delete your previous version of Windows

Applies to: Windows 10

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.

怎么样？干掉windows.old了没？如果未果，往下看?