I don't suggest to simply DELETE the windows.old in windows 10, the proper way is the following
Ref:
https://www.windowscentral.com/how-properly-remove-windowsold-folder-windows-10
How to delete the Windows.old folder on Windows 10
The Windows.old folder resides in the hard drive where the operating system is currently installed (typically the C: drive). However, you can't just hit the delete key and to make it disappear, you need to use the Disk Cleanup tool.
Use the Windows key + E keyboard shortcut to open File Explorer. From the left pane, click This PC.
Under Devices and drives, right-click the drive with the Windows 10 installation, and click Properties.
Click the Disk Cleanup button.
Click the Clean up system button.
Select the Previous Windows Installation(s) option to delete the Windows.old folder. Here you can also choose to delete other installation related files, including Windows upgrade log files and Temporary Windows installation files, which can take up several gigabytes of storage as well.
Click OK. Click Delete Files on the pop-up dialog to confirm the deletion. On the Disk Cleanup warning click Yes to complete the process.