Extension of vacation

I have a reservation Oct 3 - 5.

Today is Oct 7 obviously.

Same divers want to dive tomorrow, on Oct 8.

Calendar is not letting me extend their reservation to add the extra day. It seems to want me to create a new reservation, thus making divers complete all forms again. I’d rather not have them do that if we can avoid it.

Any way I can just extend the old reservation for the one add-on day?