Check your variables _FirstName, _LastName, _CollegeName etc are updated with recent values and RollNo field is not added to the parameters list
cmd.Parameters.AddWithValue("@Fname", _FirstName);
cmd.Parameters.AddWithValue("@Lname", _LastName);
cmd.Parameters.AddWithValue("@Clgname", _CollegeName);
cmd.Parameters.AddWithValue("@Mark", _MarksObtained);
cmd.Parameters.AddWithValue("@Exam", _Exam);
cmd.Parameters.AddWithValue("@RollNo;, ?????); // Modify this line
what is stud variable doing here? and you are not using stud var value to update statement. check that too.