You can't a Scaned Image is just that a Graphic - not in Acrobat.
There is a Graphics application called GraphicsConverter (Mac I thank they make a PC version) you can edit the Graphics.
As for Text in Acrobat You can Run OCR They say the Clearscan method works better.
With Clear Scan you get the text replacing the graphics of text. That can be edited. If you do searchable PDF conversion, you can still edit (I think). However it is an edit of the hidden layer of text behind the graphic and does not show on screen normally.
Usually the best choice is to ask the author for the original file to edit. If that is a copyright issue, then you should not be editing anyway.