The first step is to phone up the dealer as soon as possible to ask them to cancel the order and return the deposit because of the misleading information that was given. The dealer will either say "yes" or "no". If they say "no", you can consider further options which could include involving trading standards or legal action or trying to reverse the transaction through your bank.
For future reference, you need to apreciate that salesmen's main task is to sell things, or at least get a deposit and/or signature before the customer has had the oportunity to think things through properly. Being honest often comes low of their list of priorities.