If you'd like to change or delete a reservation, first go to the day and find the reservation on the days page. Behind each reservation, you'll find an Options link. Clicking it will show the available options for this reservation. This may differ a little, depending on the current state of the reservation. If it hasn't been accepted, you may accept it just to give one example.
However, you'll always find the Edit booking and the Delete booking options as the last two options.
If you delete a reservation, it will be listed at the bottom of the days page. You can always restore it from there.
Related article: Create a new reservation