This section of the guide to Destroy All Humans explains how to repeat or reset missions.
You can reset a story mission by simply stopping it and selecting the right option. This lets you start all over. Story missions are not long, so you can easily reset a task if things go south.
You can repeat all the missions in the game. All you have to do is hop over to Holopox's Dex. There, you will find all the missions available in the game.
Repeating the missions doesn't cost anything. You can do a task again at any time. It's a great way to meet your goals and score more points.