Open Settings
Go to System
Select Storage
Click Temporary files
Check Previous Windows installation(s)
Click Remove files
Open Disk Cleanup
Select the system drive
Click Clean up system files
Check Previous Windows installation(s)
Click OK
Confirm deletion
Open File Explorer
Go to the C: drive
Right-click Windows.old
Select Properties
Go to the Security tab
Take ownership if needed
Delete the folder
Open Command Prompt as administrator
Run: takeown /F C:Windows.old /R /D Y
Run: icacls C:Windows.old /grant administrators:F /T
Run: rmdir /S /Q C:Windows.old