@David2879
Go to your "Inbox" tab; click on the conversation with the prospective guest. From here, scroll down and on the left side, there will be an option to change reservation. From here, you can adjust the price accordingly.
Be certain to have all conversation with the prospective guest on the ABB platform, in case any confusion arises later.
Are you sure that you really want to rent your entire place... it could mean trouble. Have you vetted them so that you feel comfortable hosting this young couple; do they have several positive reviews, etc.
All the best!