You would do this by adding your page numbers using a Word field. The most straight-forward way of doing this is to type the word "page" without the quotation marks and then selecting that word and pressing Ctrl+F9 followed by F9. There is a keyboard shortcut for this which inserts a formatted page field: Alt+Shift+P

Finally, on the Insert ribbon tab you can find Page Numbers. Note that unless you pick the option to insert at the current position, you may be overwriting an existing header or footer.

Page Numbers in Word 2007-2013
Use automatic page numbering and let the text flow from page to page