How I can cancel an order online?

Contact the company and see if it's not too late to review the application. That has not been sent, however, you may be able to do so. If you have sent, find out what the return policy is.  

If you do not allow the return / refund, you may be able to refuse the delivery. If sent by U.S. mail, you can deny anything. Then you can make a claim in writing to your credit card company.  

If they see that the item was returned, it is likely to reverse the charge, less shipping cost. You will not be able to keep the company that if withdrew from the agreement.