There are two types of reservations. The ones that come from direct bookings. And the other ones.
On the direct bookings (manual ones or from the booking engine), you open the reservation (edit) to go inside
and click in the middle of the page, on the dates in blue to edit them. Please remember to save at the bottom of the page.
Regarding the reservations coming from OTAs, you are not allowed to update them.
The reason is that it would lead to discrepancies between Xotelia and the info on the online agencies, leading to double bookings.
Two options for you:
a. you contact the OTA in order to update that specific booking and the automatism will follow.
b. you cancel the reservation on Xotelia (no one will know) and you create a new manual reservation for the correct dates.
If it were to be modified by the OTA, you would receive an error message from Xotelia asking you to manually update the calendar on Xotelia.
Note that you can update the rate of a reservation on Xotelia the same way.