Manipulating dataset tables

DataTable firstTable = dataSet.Tables["FirstTableName"];
DataTable secondTable = dataSet.Tables["SecondTableName"];
//
//Iterate trough first data table data
foreach(DataRow row in firstTable.Rows)
{
    ModifySecondTable(rowFromFirstTable);     
}
...
void ModifySecondTable(DataRow rowFromFirstTable, DataTable secondTable)
{
  int fk = row["ID"]; // Access the ID, or the PK from first table that is FK in the 2nd one!
  foreach(DataRow secondTableRow in secondTable.Rows)
  {
      int linkField = secondTableRow["linkField"]; //Here you should put the real name of the FK!
      if(linkField == fk)
      {
          // TO DO:
          //Put your code that use the data from rowFromFirstTable to update the data from secondTableRow!
      }   
  }
}