The question really is "why are you trying to change a roll number?"
Normally, that is the "official ID" that identifies a student and all the data about him or her.
Changing that is dangerous: it can result in duplications, in data being attached to the wrong student, in all sorts of database integrity failures.
But there is a logic problem with the whole idea:
Quote:
My form contain only 1 TextBox for ROLLNUMBER.
If your form only has one roll number, then you have no value to look for to find the existing record if it contains the new number; or no value to set it to if it contains the original value which identifies the record.
You need two values; without both all you can do is change the roll number to the same value, or not find the existing record!
So FreedMalloc is right: you need to pass two different values through in order to get this to work.