You can only cancel a leave application that has NOT been approved yet. Once it has been approved, you will need to request for the approver(s) to first reject that leave application before you can edit/delete it. 

Go to My Leave > My Leave Records

Click into the leave record that you would like to cancel. Click on the blue text that says "Click here to cancel". If you only wish to edit some of the details of the leave application, you can just click on the edit icon in the top right hand corner. This will update your leave application without cancelling it.