Your problem is that the scanned document is simply a stored image, not in text form.
You have choices - use an OPR programme to convert the image to text document, save as a Word doc and then edit;
Open the scanned image in Paint and use a text tool to modify the document;
Convert to pdf and use a pdf editor.
The last option gives the best results, but I don't know of any good free pdf editor software. I use Foxit pdf editor - there may be a free trial here:
http://downloads.vnunet.com/download/text+edit ors/foxit+pdf+editor/_37089.html