BOOL writeRecord(cdbfapiPlus* handle, int recno);

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