public partial class LogintoProfile : Form
{
    public LogintoProfile()
    {
        InitializeComponent();
    }

    private void BoxPawo_TextChanged(object sender, EventArgs e)
    {
        //Boxen viser stjerner i stedet for text
        BoxPawo.PasswordChar = '*';
        //Maxlængde på Password
        BoxPawo.MaxLength = 36;
    }

    private void BoxCopawo_TextChanged(object sender, EventArgs e)
    {
        //Boxen viser stjerner i stedet for text
        BoxPawo.PasswordChar = '*';
        //Maxlængde på Password
        BoxPawo.MaxLength = 36;
    }

    private void Update_Click(object sender, EventArgs e)
    {
        string connStr = "server=localhost;user=root;database=p4_projekt;port=3306;password=Jeppesen95;charset=latin1;";
        MySqlConnection conn = new MySqlConnection(connStr);

        try
        {
           MessageBox.Show("Forbinder til databasen");
            conn.Open();
            string sql = "UPDATE p4_projekt.customer_table set First_name='" + BoxFornavn.Text + "',Last_name='" + BoxEfternavn.Text + "',Email='" + textBoxEmail.Text + "',Password=" + BoxPawo.Text + "WHERE Customer_id=@id";
            MySqlCommand cmd = new MySqlCommand(sql, conn);
            MySqlDataReader rdr = cmd.ExecuteReader();

            while (rdr.Read())
            {
                Console.WriteLine(rdr[0] + " -- " + rdr[1]);   // [] kan ikke huske om det er array plads?!
            }

            rdr.Close();
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.ToString());
        }

        conn.Close();
        Console.WriteLine("Done.");
        Console.ReadLine();
    }

    private void TankOp_Click(object sender, EventArgs e)
    {
        string connStr = "server=localhost;user=root;database=p4_projekt;port=3306;password=**********;charset=latin1;";
        MySqlConnection conn = new MySqlConnection(connStr);

        try
        {
            MessageBox.Show("Forbinder til databasen");
            conn.Open();
            string sql = "UPDATE p4_projekt.customer_table set Balance'" + Balance.Text + "WHERE Customer_id=@id";
            MySqlCommand cmd = new MySqlCommand(sql, conn);
            MySqlDataReader rdr = cmd.ExecuteReader();

            while (rdr.Read())
            {
                Console.WriteLine(rdr[0] + " -- " + rdr[1]);   // [] kan ikke huske om det er array plads?!
            }

            rdr.Close();
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.ToString());
        }

        conn.Close();
        Console.WriteLine("Done.");
        Console.ReadLine();
    }

    private void Back_Click(object sender, EventArgs e)
    {
        this.Hide();
        Login.Login name = new Login.Login();
        name.ShowDialog();
    }
}

public partial class Login : Form
{
    private string connStr;
    private MySqlConnection conn;

    public Login()
    {
        InitializeComponent();
    }

    private void connect_to_DB()
    {
        try
        {
            connStr = "server=localhost;user=root;database=p4_projekt;port=3306;password=**********;charset=latin1;";
            conn = new MySqlConnection(connStr);
            conn.Open();
        }
        catch (MySqlException e)
        {
            throw;
        }
    }

    private bool login_validation(string email, string pass)
    {
        connect_to_DB();

        MySqlCommand cmd = new MySqlCommand();
        cmd.CommandText = "Select * from customer_table where Email=@email and Password=@pass";
        cmd.Parameters.AddWithValue("@email", email);
        cmd.Parameters.AddWithValue("@pass", pass);
        cmd.Connection = conn;

        MySqlDataReader login = cmd.ExecuteReader();

        if (login.Read())
        {
            conn.Close();
            return true;
        }
        else
        {
            conn.Close();
            return false;
        }
    }

    private void buttonSubmit_Click(object sender, EventArgs e)
    {
        string email = textBoxEmail.Text;
        string pass = textBoxPass.Text;

        if (email == "" || pass == "")
        {
            MessageBox.Show("Tomme felter, udfyld venligst begge felter");
            return;
        }

        bool r = login_validation(email, pass);

        if (r)
        {
            MessageBox.Show("Korrekte oplysninger");
            this.Hide();
            LogintoProfile name = new LogintoProfile();
            name.ShowDialog();
        }
        else
            MessageBox.Show("Forkerte oplysninger");
    }
}

UPDATE ...
WHERE Email = @Email

string sql = @"UPDATE p4_projekt.customer_table
               set First_name=@firstname, Last_name=@lastname,
                   Email=@email,Password=@password
               WHERE Customer_id=@id";
  MySqlCommand cmd = new MySqlCommand(sql, conn);
  cmd.Parameters.Add("@firstname", MySqlDbType.VarChar).Value = BoxFornavn.Text;
  cmd.Parameters.Add("@lastname, MySqlDbType.VarChar).Value = BoxEfternavn.Text;
  cmd.Parameters.Add("@email, MySqlDbType.VarChar).Value = textBoxEmail.Text;
  cmd.Parameters.Add("@password, MySqlDbType.VarChar).Value = BoxPawo.Text;
  cmd.Parameters.Add("@id, MySqlDbType.Int32).Value =  ???? // no value for id ???