Once it's booked, there's a penalty for cancellation. It would be best to uphold it and fix the other pricing on your calendar to reflect the prices you meant to charge. You could also try messaging the guest directly about it to see if they would agree to this price change, but I think it would be risky in terms of the rating they could give. If you did want to try and they did agree, you could charge for the additional funds through the resolutions center, airbnb.com/resolutions