When you click on the reservation it will open in a popup.
You can also click on the reservation from the Reservations List page and from the Calendar to get the same view.
From there you see options very clearly but here are explanations below:
You can update the reservation status which will change also the color of the reservation in the calendar so you have detailed overview of all reservations on busy days
View Credit Card details
Create Invoice (documentation for invoice)
Delete it (cancel) if it is Private reservations
You can also EDIT reservations and then you get to the same windows as if you are creating a new reservation so you can change any detail you wish (check the documentation here)
And you can also Mark Reservation as a No-Show or Invalid Credit Card if the reservation is from Booking.com so you don't have to open their Extranet in order to accomplish these actions.