Hi Joana and Pedro,
You can alter it by going to your message and open your guest's message.You will see "Change/Cancel" reservation.Click on that one the another option will come up "Change reservation" or "Cancel reservation".Choose "CHANGE RESERVATION".You can adjust how many days they want to stay and how many guests are staying.Then hit SEND.Your guest will get this message and ask your guest to accept the changes.The guest's payment will also go up if you have fee for an additional guest.Try to use your laptop instead of your cellphone when making a changes. Make sure you choose CHANGE RESERVATION and NOT cancel reservation.
Hope this helps
Leah