If I understand your question correctly, you can't do that. If you want to take a finished video which includes "super" text (text on top of the video) and you do not have the separate video file without the text available, it is not practical to change the text to be different text. You can blur it or cover it with a colored box, but you can't change it to be different letters.
If you want to change subtitles, on the other hand, many video packages can do that. If so, please edit your question to explain exactly what you want to do.
EDIT: Having read your comments: almost any video editor can blur a portion of the screen. My former employer (with which I have no current business relationship) has an article on their web site that might be helpful: http://www.deskshare.com/resources/articles/vem_watermark_videos.aspx