IF you have not deleted the windows.old from C drive,
and,
IF you installed your win 10 less than 30 days ago.
If yes to BOTH of the above, you can go back to your Win 8.1 by doing this.............
Left click at Start > left side, click Settings > select Update & Security > left side, select Recovery > right side, under Go back to Windows 8.1, click the Get started button.
If you cannot do the above, you will have to reinstall Win 8.1