Going through the probate process in court, if the deceased person left a will and the vehicle was not specifically devised to someone else.
Providing proof of heirship, such as a death certificate and an affidavit of heirship, if there is no will.
Going to the local Department of Motor Vehicles office (DMV) in person and fill a form REG 256 and REG 5.
Once the heir has a title to the vehicle, they can sell it by:
Selling the car privately, by advertising it online or through word of mouth.
Trading in the car at a dealership.
Selling it to a car buying service.