bool writeField(int recno, int fieldno);

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

{
    // your code
    for( int row = 0; row < dbf.recCount()-1; row++ )
    {
        dbf.readRecord(row);
        // change data
        dbf.writeField(row, field);
    }
}