Suppose you're in Level 1, the mission where you have to destroy Mr. Smithers' car with the Plow King. During this mission, all you have to do is drive to a phone booth and get out. A 10 second timer should start. Before the timer runs out, call another car you want to use, such as the Family Sedan, from the booth. The timer will run out, and you'll get the "Retry Mission" screen. Select yes, and you'll be able to repeat the mission, but in the car you called from the booth instead of Plow King.