Did you have a couple Mission participants who weren't able to respond as fast as you would expected? Or do you want to give people a couple extra days as you launched the mission during a holiday week? Just click "Extend Mission" and change the end date!
Once a mission has started, if it was launched with an end date, you will see the option to "Extend Mission".
By default it will add exactly one day from the original end date, but you can choose any day from the calendar picker and then click "Save new end date".
Then your new end date for the mission is good to go!