Win 10 itself does not occupy too much root disk space, which is below 20G. However, when upgrading, it will cause the residue of the previous preview version, especially the recent versions have changed a lot, which is basically a brand-new installation, so it will automatically generate the windows.old directory for rollback.

This directory is also around 10G. If you forcibly delete it with the right button, you will get an error that the system permissions are insufficient and cannot be deleted. How to delete it?

In My Computer, right-click the drive letter where the directory is located. For example, Bian Xiao is on drive C and selects the bottom line in the pop-up menu.

Select the tab, and then select

The system will automatically scan and collect the space that can be cleaned, but the system will not clean the system files by default, so you need to choose as shown in the red circle in the figure below.

The system will calculate the space again, this time it will calculate the old system files. choose

The system will show that the files on drive C are being cleaned, so that after cleaning, there will be nearly 13G space on the root disk, which is really slim.