How to show values in textboxes when combobox value select in c# windows application using layer architecture

Question

I have a code to show combobox value in textboxes and code work fine for me but I convert my project into layer architecture (UI, BLL, DAL, Entity). I don't know c# very well and I'm learning the c#. My problem is that I want to change my code into layer architecture but I don't know how to do that, please help me. Here is the code.

    using (SQLiteConnection conn = new SQLiteConnection("Data Source=combolist.db;Version=3;"))
            {
                string CommandText = "SELECT * FROM combo WHERE [Id]=@id";
                using (SQLiteCommand cmd = new SQLiteCommand(CommandText, conn))
                {
                    cmd.Parameters.AddWithValue("@id", comboBox1.SelectedItem.ToString());
                    conn.Open();
                    cmd.ExecuteNonQuery();
                    DataTable dt = new DataTable();
                    SQLiteDataAdapter da = new SQLiteDataAdapter(cmd);
                    da.Fill(dt);
                    foreach (DataRow dr in dt.Rows)
                    {
                        textBox1.Text = dr["Id"].ToString();
                        textBox2.Text = dr["FirstName"].ToString();
                        textBox3.Text = dr["LastName"].ToString();
                        textBox4.Text = dr["Age"].ToString();
                        textBox5.Text = dr["Address"].ToString();

                    }
                }
            }