How do I edit a scanned document

If you are talking about text, once a document has been scanned, it's stored as an image - a bunch of pixels, it's no longer text. You need OCR software (Optical Character Recognition), that looks at an image and tries to interpret the pixels as text. It will make mistakes, though. So it would need a thorough read through.

It would be better to trace the original document, or to store files as PDFs, in which case text is still retained as text, and plugins for Acrobat (such as Pitstop Pro) can be used to edit data therein.

You open the document in whatever application that is capable of editing documents in the format it was scanned in.