func writeField(_ recno: Int, fieldno: Int) -> Bool

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 in 0..<dbf.recCount()
    {
        dbf.readRecord(row)
        // change data
        dbf.writeField(row, fieldno:field)
    }
}