To edit a scanned document you'll need to have OCR software (Optical Character Recognition.) A scanner stores the document as an image, as though it took a photograph of the document, and OCR software recognizes the text in a scanned image and makes the text editable.
Good quality OCR software, like ABBYY Finereader, is expensive. If you're planning to edit a lot of scanned documents, or if you require the highest possible degree of accuracy, then the price will be worthwhile.
On the other hand, if you only need to do this occasionally then you can use one of the free OCR converters available on the internet. For example: If you scanned your document to PDF format and you want to edit it, search the internet with this search term:
convert PDF to DOC
and your search results will include free OCR converters from PDF format to DOC format.