Get the link of your newest blast from the archive page by clicking the link for the latest title & then copying the url from the url bar at the top of your browser with your mouse
Edit your Newsletter page in the admin Admin > Pages, click on the “Newsletter” page. Edit it in TEXT mode, not visual by clicking the “text” tab at the top right of the page edit box.
Change the url inside the iframe (shown in bold below) to be the new url. MAKE SURE to leave both quotes around that url or it won’t work.