Ask the new car dealer if he will allow you to back out of your contract. He does not have to let you do this, but it does not hurt to ask, especially since there may be no other way out.
Consider whether or not a fraud may have been perpetrated against you. If the new car dealer has misrepresented or failed to disclose an important fact concerning the car you have contracted to purchase, you may be able to cancel the new car deal.
Consider whether or not your state's "lemon law" may apply, to see if you can cancel your new car deal under this exception to the general rule. In most states the new car buyer can do so if a major defect covered under warranty is discovered in the car and the defect remains unfixed after a reasonable number of repair efforts have been completed by the dealer or manufacturer.