function writeRecord(recno : Integer): Boolean;

Function writeField() writes a single field from internal memory block to the file.
You have to fill the memory with necessary values using setValue()

begin
    // your code
    for row := 0 to dbf.recCount()-1 do
    begin
        dbf.readRecord( row );
        // change data
        dbf.writeField( row , field );
    end;
end;