There are two ways to add page numbers: 1) adding it to the top or bottom of the page, or 2) inserting it in the current position. Both options are available from the Insert ribbon > Page Number button or the Header/Footer Tools ribbon > Page Number button.
While the first method may seem easier, there are significant problems with it. First, Word adds an unnecessary, extra hard return after the page number, which you’re going to have to delete. Second (and most importantly), anything that was previously typed in the footer or header will be deleted. You will have to re-create that content. This is typically a problem for document IDs, file paths, initial blocks, document titles and letterhead.