mysql> create table DemoTable1985
   (
   DueDate varchar(20)
   );
Query OK, 0 rows affected (0.00 sec)

mysql> insert into DemoTable1985 values('Jan 2017');
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1985 values('May 2018');
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1985 values('Aug 2015');
Query OK, 1 row affected (0.00 sec)

mysql> select * from DemoTable1985;

+----------+
| DueDate  |
+----------+
| Jan 2017 |
| May 2018 |
| Aug 2015 |
+----------+
3 rows in set (0.00 sec)

mysql> select date_format(str_to_date(DueDate,'%b %Y'), '%Y-%m-01') as Result from DemoTable1985;

+------------+
| Result     |
+------------+
| 2017-01-01 |
| 2018-05-01 |
| 2015-08-01 |
+------------+
3 rows in set (0.00 sec)