You can update a variety of guest and reservation details simply by clicking on the reservation in the calendar to access the Reservation Details. See more information here.
Keep in mind that for channel bookings, changes made to the reservation itself in Guesty For Hosts, like check-in/out date or number of guests, are not synced back to the channel. These edit functions should only be used for direct or manual bookings. See here for more details about channel management.