You cannot edit a tweet. You can copy a tweet and repost a revised version of it. It will then have a new time stamp.You can delete a tweet entirely. There are apps like Brizzly that allow you to set a delay of up to 10 minutes in which you can edit a tweet before it posts but after it posts, it cannot be edited. Brizzly also has an autodelete function that will delete your tweet automatically after one day, one week, or one month, and it will let you save your favorite tweets from deletion.