In Word, head to File > Open and then navigate to the PDF file that you'd like to edit. Word will automatically convert the PDF into an editable Word document. Once it opens, make any edits you need to.
Why can't I edit a PDF in Word?
You can't edit that PDF in Word until OCR has been done. Optical Character Recognition looks at the page and tries to figure out whether there are words. If it's typed and neat, OCR can be very accurate. If it's handwritten or has marks, OCR can be just awful.