@Madelyn50
The guest can change the dates on the reservation, but the new dates will reset the cancellation window and they could cancel the new reservation for the future dates and will be refunded according to your cancellation policy. You should decide if you want to offer the same price for the new dates and ensure the price is correct on your calendar. It is better to have the guest send the request to you and you can accept it.
Some Hosts will not allow date changes into the future for reservations for this reason. They have the guest cancel the original stay and then offer a discount for a future stay.
Changing the Dates on a Reservation (Guest)
https://www.airbnb.com/help/article/913
Responding to Guest's Trip Change
https://www.airbnb.com/help/article/1504