ALTER TABLE yourTableName change yourColumName yourColumnName dataType AUTO_INCREMENT PRIMARY KEY;

mysql> create table AlterTableToAddAutoIncrement
   -> (
   -> StudentId int
   -> );
Query OK, 0 rows affected (0.57 sec)

mysql> alter table AlterTableToAddAutoIncrement change StudentId StudentId int AUTO_INCREMENT Primary  key;
Query OK, 0 rows affected (1.93 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> insert into AlterTableToAddAutoIncrement values();
Query OK, 1 row affected (0.16 sec)
mysql> insert into AlterTableToAddAutoIncrement values();
Query OK, 1 row affected (0.16 sec)
mysql> insert into AlterTableToAddAutoIncrement values();
Query OK, 1 row affected (0.33 sec)

mysql> select *from AlterTableToAddAutoIncrement;

+-----------+
| StudentId |
+-----------+
|         1 |
|         2 |
|         3 |
+-----------+
3 rows in set (0.00 sec)