@Debbie815 Go to the "Reservations" tab in your dashboard, select "Change or Cancel," and manually enter the altered checkout date. Here you'll have a chance to choose a new total price (excluding Airbnb fees, which will be charged to the guest on top of your rate). When you complete this process, your offer will be sent to the guest, and only becomes valid when they accept.
One thing to watch for - if the guest does not have funds in their original payment method, you may find yourself in a situation where the calendar dates are blocked but the booking is unpaid. This can quickly become a complicated situation. So I suggest asking your guest to make sure their user account's payment method is valid and funded before accepting the date change request.