@Stephanie1012 @Alice-and-Jeff0 @Ben551 I may be wrong, but I think the links mentioned above are only useful if the reservation hasn't already started.
This also happened to us the other day, where they booked for 2 guests and turned up with 3.
I tried changing the reservation by using the guide from the link above, as I have done before..... but it wouldn't allow me to change a reservation once it had already started.
I'm so sorry that I don't have an answer for you Stephanie..... but I'm also curious as to what I should have done.
In our case, we just didn't change the booking, and they paid in cash for the extra person.... but this wasn't ideal, and I wasn't really comfortable with this, as Airbnb require all guests to be registered for insurance purposes.
Please keep me posted if you find out how.