MySqlConnection coon4 = new MySqlConnection(connStr);
            coon4.Open();
            MySqlCommand command4 = new MySqlCommand();
            command4.Connection = coon4;//cn твой SqlConnection
            command4.CommandText = "UPDATE Event SET Date = @dateValue where EventName = @eventName";
            command4.Parameters.AddWithValue("@dateValue ", dateTimePicker2.Value.ToShortDateString());
            command4.Parameters.AddWithValue("@eventName", textBox1.Text);
            command4.ExecuteNonQuery();