Have a look on auto trader for rough prices. If you like what you see then sell, if you dont, keep it.
If you bought it outright then it cousl release some funds for you or alternatively if you bought on finnace it could release some additonal cash on a monthly basis.
BUT what ever car you replace it wth will be cheaper, less well equipped (unless it is a high spec older car) and you wont enjoy it as much.
if i were you i would ask myself one question...Do you need the money it will release?
Stu