Since the text is part of the video, you cannot edit existing text. If you have saved a project of your movie, then you can open the project, make changes to the text and then make another movie.
If the text is a watermark on the video, then you can try overlaying it. You may find the following post useful:
http://www.deskshare.com/Forums/ShowPost.aspx?PostID=636
DeskShare - Support