You may get a trouble that the unused space on C was run out after a Windows update. Actually, it is caused by the Windows.old folder. Windows make a backup to the Windows.old folder before updating your system and the folder has not been deleted after the Windows updating.
This article shows you the exact steps to delete the Windows.old folder for releasing the space on your C partition.
1. Press Win+R key at the same time to open the Run.
2. Enter Cleanmgr.exe and click the “OK”.
3. Select the C drive you want to clean up and click “OK”.
4. Tick the option “Previous Windows Installation(s)” and click “OK” to remove it.
5. Now, the Windows.old folder has been removed from your C partition.