There isn't usually an easy way out of a finance agreement.
If you hand the car back, it will be sold at the next car auction in your area, perhaps without getting a wash, to a dealer at "trade" price. The proceeds will be taken of the outstanding balance on the agreement and "admin" charges will be added. You will then asked to pay the new balance in one payment.
If you sell the car (with the finance company's consent), at least you can wash it first and get a better price. You would also avoid some of the admin fees.
Whatever the car sells for, you are responsible to pay off the agreement.