It's not that easy. There are two ways, neither very much fun. One way is to see if the bank or company that loaned you the money will allow them to assume the same loan in their own name, which would be just as much of a troublesome loan process as if they borrowed money to buy it outright from you.
The second option would be to just hand them the payment book and trust them to make the payments on time. Unless it's someone you know well, I wouldn't do that. The last person I know who did ended up bankrupt when they were sued after the person stopped making payments and they didn't know it.
You'd be better off to just try to sell it in the Auto Trader for what you owe on it and hope someone bites. Good luck.
