The way to cancel a leave request is to kill it (Admin only). Firstly you will need to know who the leave is for and the numerical part of the reference number and then go to Admin > System Functionality > Leave > Kill Record.
Leave that is in the future can be canceled by going to People > employee name > Leave > Leave and select the Absence Ref and change the approval decision from Approved to Cancelled. If the leave isn't approved yet you can just reject the request to cancel it.
Once cancelled you can add it back as it should be.