What a dilemma. If there is nothing wrong with the vehicle, and you simply have just changed your mind this is what car dealerships call a "cooling off period". It's usually posted on signs throughout the dealership that there is a "No Cooling Off Period" policy. This may mean that you are stuck with the car.
However, a professional sales manager may be able to work with you just days after the purchase to help you find the perfect car for you - of their on-site inventory. Once papers are signed and financing is in place, it's near impossible to cancel the transaction.
Lastly, some dealerships and auto makers offer a 1,000 mile 'test drive' with an opportunity to return the car risk-free.
Best advice, speak to the sales manager at the dealership about your options.
California has just such a 'cooling off period.' If only the citizenry would rally for a fight with their individual states for a right that they should possess, the laws could be changed. Bug the hell out of all of your government officials until this issue is before the people of the state. This is how many used-cars dealers snooker good and honest people like yourselves, and every state seems to be on the side of the crooked car dealer. UNFAIR! Let's band together and stop this nonsense. You can return a toaster, but not a car that you were lied to about? I was sold a rebuilt Ford with screws missing, but I had no say in the matter once I drove off the lot, this is evil, and it must be stopped.