Some states have a minimum cooling off period that allows a small period of time for a consumer to return certain items for a refund. The span of time is usually 3 or 5 days. It may be hard to support your position to cancel the contract when you have been aware of the issue for such a period of time and have benefited from the use of the chattel, which is now presumably depreciated. I think the relief that you should be seeking would be to enforce the terms of the contract to which you actually agreed. You should speak to a consumer lawyer in your jurisdiction. There may be laws in your state, in addition to federal laws, in place to protect you from this kind of activity.
This is not legal advice. Seek a consultation with an attorney in your jurisdiction. An attorney/client relationship is not sought or formed by the participation in this website. I am licensed to practice law in New York and New Jersey.