IT IS NOT POSSIBLE TO EDIT THE DAYS OF A LEAVE REQUEST ONCE THE REQUEST IS IN DRAFT, AWAITING APPROVAL OR APPROVED.
The best way to edit a leave request is to cancel it and re-add it. As an Admin the easiest way to do this is to kill it. Firstly you will need to know who the leave is for and the numerical part of the reference number 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 another leave request as it should have been.