I just a guest who needed to do this today. What do you have set under Listings/Booking Settings/Calendar/Availability Settings/Availability window? You should be able to set this to “All Future Dates”, on the app and on the desktop.
Also, if you want to change the dates of a reservation past the current dates, it’s best to call Airbnb to do it. Otherwise, a guest can just cancel it once it is moved, because it will be a “new” reservation in which cancellation is free for the first 48 hours. Airbnb can change the booking in such a way so that the guest can’t use this loophole.