dataTable.Columns["Qty"].SetOrdinal(0); 
dataTable.Columns["Unit"].SetOrdinal(1);

public static class DataTableExtensions 
{ 
    public static void SetColumnsOrder(this DataTable table, params String[] columnNames) 
    { 
        int columnIndex = 0; 
        foreach(var columnName in columnNames) 
        { 
            table.Columns[columnName].SetOrdinal(columnIndex); 
            columnIndex++; 
        } 
    } 
}

table.SetColumnsOrder("Qty", "Unit", "Id");

table.SetColumnsOrder(new string[]{"Qty", "Unit", "Id"});