You can't directly edit text on a scanned picture.
It's possible to extract text from a scanned image using OCR software (which I seem to recall has been mentioned before in response to one of your posts) but that doesn't retain the image.
So what can you do? Let's assume that you've got a photo of a branch of W H Smith and you want to change the name of the shop to Steg's Megastore. You can't do it by editing the text directly. You need to do what a sign writer would do, i.e. paint over it and put your own text on top. To do that you need an image editing program. Photoshop would be ideal but it's far from cheap. However there's a free alternative (GIMP) which is nearly as good. Download GIMP 2.6.7 from here:
http://gimp-win.sourceforge.net/stable.html
(NB: I strongly recommend that you should download the help file as well. Like Photoshop, GIMP takes a while to fully get to grips with).
Chris