Using Datareader value to query database

Am a new programmer in c# and i want to use the value return in datareader to re-query the database.
This is the part of my code:

 string queryscore = "SELECT CourseUnit,Score,MatricNo FROM student_result WHERE CourseCode='" + cmbcoursecode.Text.ToString() + "'";
 
            SqlCommand cmdscore = new SqlCommand(queryscore, con);
 
            SqlDataReader readerscore;
 
            readerscore = cmdscore.ExecuteReader();
 
            while (readerscore.Read())
            {
                score1 = (int)readerscore["Score"];
                int CourseUnit = (int)readerscore["CourseUnit"];
                if (score1 <= 39)
                {
                    string updateresult = "Update Student_Result SET Grade='" + "F" + "' WHERE MatricNo='" + readerscore["MatricNo"] + "'";
                    SqlCommand cmdupdatescore = new SqlCommand(updateresult, con);
                    cmdupdatescore.ExecuteNonQuery();
                   // MessageBox.Show("F");

                }