I cant change constraint,, can you give me sub query
No body is asking you to change any constraint. You have a wrong design and you are being asked to correct it. This clearly should be Foreign Key candidate.
Instead of trying to update/correct the rollno field in both tables everytime, you should fix the table design and make sure that main table rollno is a foreign key of tablesecond.
Still, if you want to update roll no from table 1 to table 2, then first you need to get the missing rollno in table2 and then insert them.
Select Rollno FROM mainTable Where rollno NOT IN (Select Rollno from TableSecond)
Either in SP or code, you need to loop through these rollno to insert and match the two tables.