Are you sure, you want to delete reservation? If you want to change reservation details, please continue on Edit reservation.
If the reservation is not valid anymore, you can delete it. But firstlevel make sure, there wasn't online payment. In that case you should return money back or issue cancelation fee.
When the reservation was done on online booking engine, your guest can online cancel the reservation.
Set up time, until when can guest cancel the reservation manually.
Click on main menu - Settings - System - Online settings