private void button6_Click(object sender, EventArgs e)
{
    try
    {
        connection.Open();
        OleDbCommand command = new OleDbCommand();
        command.Connection = connection;
        string query =
            "UPDATE Marc SET Marcstud =@ln  Where IDSubject=@subject AND IDStudent=@student";

        //command.Parameters.AddWithValue("@ln", Convert.ToInt32(textBox_m.Text));
        //command.Parameters.AddWithValue("@student", Convert.ToInt32(textBox_S.Text));
        //command.Parameters.AddWithValue("@subject", Convert.ToInt32(textBox_p.Text));

        command.Parameters.AddWithValue("@ln", 4);
        command.Parameters.AddWithValue("@student", 301);
        command.Parameters.AddWithValue("@subject", 101);


        MessageBox.Show(query);
        command.CommandText = query;

        command.ExecuteNonQuery();
        MessageBox.Show("data edit successful");
        connection.Close();
    }
    catch (Exception ex)
    {
        MessageBox.Show("error" + ex); ;
    }
}

public void Update(String connectionString)
{
    var ln = Convert.ToInt32(textBox_m.Text);
    var student = Convert.ToInt32(textBox_S.Text);
    var subject = Convert.ToInt32(textBox_p.Text);

    using (var connection = new OleDbConnection(connectionString))
    {
        using (var command = connection.CreateCommand())
        {
            command.CommandText = "UPDATE Marc SET Marcstud = @ln Where IDSubject = @subject AND IDStudent = @student;";
            command.Parameters.AddRange(new OleDbParameter[]
            {
                new OleDbParameter("@ln", ln),
                new OleDbParameter("@student", student),
                new OleDbParameter("@subject", subject)
            });

            connection.Open();
            command.ExecuteNonQuery();
        }
    }
}