In order to cancel a withdrawal, access your account and select the 'User' icon at the top of our page. Go to 'Withdrawal' next. A list of all pending withdrawals will appear. If you see a withdrawal, which is not processed yet, you can easily return it by hitting the 'Cancel' button.
Once a withdrawal is approved it cannot be returned anymore. Further, it will also disappear from the list of pending withdrawals. Nevertheless you can do a new deposit anytime in such case.