Before updating the document, make sure you have uploaded a new version of the file - follow this video.
To update a document viewed within a page using the "File Viewer" content type you'll first need to go to the page the file is located on. Click Edit at the top right corner of the window. Select the content area where the old document is located. Then click Edit (pencil symbol) a the left of the content window. Click Change File, choose the new file and click Insert, then click Apply at the bottom of the window. You can now close that dialog box by clicking the X. Finally, click Republish.