You might not be able to, since the general rule, unless the contracts themselves say otherwise, is that contracts are binding and enforceable and can't be cancelled. There's no 3 day "coolling off" period for used cars. The dealer may have even had signs on their dealership's walls saying so.
Here, it doesn't matter that the previous lender didn't approve you. And your month of using the car as well as your trade in car and your signed documents probably provided the "consideration" that makes the contract binding.
But as is true for any question that asks "what are my contractual rights?" no lawyer can give any meaningful answer without reviewing the documents in question.
See a lawyer for help, and in the future, obviously, don't sign anything you haven't read thoroughly and understood and agree with completely, and don't assume that contracts you sign aren't enforceable.
PLEASE READ THIS BEFORE YOU COMMENT, EMAIL ME OR PHONE ME. I'm only licensed in CA. This answer doesn't make me your lawyer, and neither do follow-up comments and/or emails and/or phone calls, and you shouldn't expect me to respond to your further questions if you haven't hired me. We need an actual agreement confirmed in writing before any attorney-client relationship is formed. This answer doesn't constitute legal advice, and shouldn't be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue.