Datatable C# Input string was not in a correct format

Expected Type as Int32

If gtennha contains a number, you can parse it to an int. With more safeguards:

var gtennha = cmd.ExecuteScalar();
try 
{
   if(gtennha != null)
      dt.Rows[i][1] = int.Parse(gtennha.ToString());
   else 
      Console.WriteLine("Error: Query didn't return a result.");
} 
catch(FormatException ex)
{
   Console.WriteLine("Error: Couldn't parse 'gtennha' to a number.");
   /* more error handling */
}

Parse string to int

DataTable dt = new DataTable();
        mySqlDataAdapter.Fill(dt);
        int socot = dt.Rows.Count;
        for (int i = 0; i < socot; i++)
        {
            String dn = dt.Rows[i][1].ToString();
            String gduan = "SELECT tennha FROM duannha WHERE id=@id";
            MySqlCommand cmd = new MySqlCommand();
            cmd.Connection = conn;
            cmd.CommandText = gduan;
            cmd.Parameters.AddWithValue("@id", dn);
            String gtennha = cmd.ExecuteScalar().ToString();
            int a;
            int.TryParse(gtennha, out a);
            dt.Rows[i][1] = a;
        }

Update

if gtennha always start with VIOFFICE or a word something like that then use this way split string

DataTable dt = new DataTable();
        mySqlDataAdapter.Fill(dt);
        int socot = dt.Rows.Count;
        for (int i = 0; i < socot; i++)
        {
            String dn = dt.Rows[i][1].ToString();
            String gduan = "SELECT tennha FROM duannha WHERE id=@id";
            MySqlCommand cmd = new MySqlCommand();
            cmd.Connection = conn;
            cmd.CommandText = gduan;
            cmd.Parameters.AddWithValue("@id", dn);
            String gtennha = cmd.ExecuteScalar().ToString();
            int a;
            int.TryParse(gtennha.Split(' ')[1], out a);
            dt.Rows[i][1] = a;
        }

I think you have choosen the wrong column to assign the return value of your ExecuteScalar query. From your error message it is clear that the return value is a string "VI BUILDING 29BD" (or something like that) and you are trying to assing that value to the same column dt.Rows[i][1] that you have used as the source for the ID parameter. This of course cannot be correct.

So you need to identify what is the column that you want to assign the return value of the ExecuteScalar and take extra care on handling that return value that could be NULL if there is no match on the WHERE clause

DataTable dt = new DataTable();
mySqlDataAdapter.Fill(dt);
int socot = dt.Rows.Count;
for (int i = 0; i < socot; i++)
{
    String dn = dt.Rows[i][1].ToString();
    String gduan = "SELECT tennha FROM duannha WHERE id=@id";
    MySqlCommand cmd = new MySqlCommand();
    cmd.Connection = conn;
    cmd.CommandText = gduan;
    cmd.Parameters.AddWithValue("@id", dn);
    object result = cmd.ExecuteScalar();

    // SUPPOSE THAT YOU WANT TO ASSIGN AT COLUMN 2 NOT STILL AT COLUMN 1
    if(result != null)
         dt.Rows[i][2] = gtennha;
}