Currently, modifications of confirmed bookings are no longer possible once the guest has paid for the reservation.
The only way you can modify an accepted reservation is to cancel it and make a new one that takes into account the desired changes.
Here's an article explaining how to cancel an accepted reservation.
Once cancelled, simply ask your traveler to send a new booking request with the desired changes.
Please note that you should always contact the traveler prior to modifying a reservation.
As WeChalet grows, we will soon be offering new features to modify an existing reservation.