For now, only team members having authority to accept leave request can cancel a leave request after it is approved. For this, he/she needs to login to his/her account in LeavesOnBot+ web application.
After successful login, go to Leave Record section from the sidebar.
Now click on approved leaves. Here you will see the leaves approved. For the days for which leaves have not yet taken, you will see a Cancel button. Clicking this button, you can cancel a leave request already approved.