delete from yourTableName where yourColumnName='\\';

mysql> create table DemoTable
   (
   FolderName varchar(100)
   );
Query OK, 0 rows affected (0.67 sec)

mysql> insert into DemoTable values("????");
Query OK, 1 row affected (0.44 sec)
mysql> insert into DemoTable values("\\");
Query OK, 1 row affected (0.13 sec)

mysql> select *from DemoTable;

+------------+
| FolderName |
+------------+
| ????       |
| \         |
+------------+
2 rows in set (0.00 sec)

mysql> delete from DemoTable where FolderName='\\';
Query OK, 1 row affected (0.63 sec)

mysql> select *from DemoTable;

+------------+
| FolderName |
+------------+
| ????       |
+------------+
1 row in set (0.00 sec)