Backing out of car purchase

[Coralgurl]

Anyone know at what point you can back out of a car purchase with a dealership? Or decide on a different car if deposit paid and approval given? Ive done something I think I'm going to regret, I haven't signed the papers yet as I'm to pick up the car tomorrow. I know this is maybe not the forum to ask but this forum holds a wealth of knowledge and different backgrounds so hoping someone will know.
Thanks