Hello @Emma141,
The prices and other terms that were on you listing at the time the booking was made are the ones that apply to the booking.
If you change something after a booking the change won't apply to that booking but will apply to bookings made after the change.
As you are new and have instant booking turned on your best bet is to contact Airbnb and voice your concerns to them.
Don't cancel the booking yourself without contacting Airbnb,
Plead your case. They may be understanding of new hosts and I think they're also more likely to consider accommodating the guests elsewhere for a first time problem, when you had instant booking on.
Contact information, as well as lots of other helpful information, can be found in the "Community Guidelines" post you'll see if you click on the "get support" link on the main page of this community forum.
As a shortcut here's the contact article.
Contact-Airbnb-A-Community-Help-Guide
Steve.