Hi @Paulo52,
Although this is a somewhat older post, just a little reaction. So it may come in handy to some other hosts out here.
You or the guest, should have gone into the existing reservation (any day before the departure day). And change the reservation with the additional days extension. When the other party accepts, within 24 hours. The system will take care of the rest, such as additional charge to the guest and payment to you.
Simple.
Just putting the wish out it in writing or verbally, is definitely not enough!
Hope this helps.
Success!