A user can withdraw leave in the future by clicking the Absence Ref in the 'My Future Leave Requests' list on their Dashboard.  At the bottom of the leave request is a Withdraw Request button.

OR

As a line manager (i.e. a Manager of a Group) they can find their direct reports by going to My Details > Groups and click on an employee.

As a Client Admin (the access you have) or Management Site Role you can access an employee simply by going to People > employee name.

...From there go to Leave > Leave and click on the Future leave request to cancel.  Click the Approved link to open the decision box and change the decision from Approved to Cancel Request and Submit.

In some cases you will want to cancel leave that falls in the past and to do that you must be a Client Admin and go to Admin > System Functionality > leave > Kill Record.  You will need to know the person its for and the numerical part of the Absence Reference (e.g. Ar001234 = 1234).  This extra control on past leave gives you more security in the system.