I just bought a car and now the company is trying to change the deal after everything has been signed...?

I just bought a car and had it for just a week. The company was fine all the paper work was in order and signed off on. Now they are saying I need a co-signer or I have to give the car back. Can they do this with everything being signed and me having the car for this amount of time? The already have my down payment as well...