@Ivana5 This type of mistake is unfortunately all to common and really infuriates guests. Your only option is to keep the reservation at the price you had on your calendar or cancel it with all the attendant penalties including an automated review which says you cancelled a reservation. That review will be there for all prospective guests to see which may cause you lost income. Also, keep in mind that the dates for the cancelled reservation will remain blocked by Airbnb so you won't be able to rebook them.
If you are going to allow people to book far off in the future, you must be on top of your pricing for those dates! Asking guests to pay additional money at the time of arrival is against Airbnb's terms of service. If the guest tells Airbnb about this, they could remove your listing.
Bottom line: Take this as a very expensive lesson learned and keep your pricing up to date!!