There may be settings for this in your admin (look under Settings or Appearance > Customize), but it’s not likely. In order to change it then, you’ll need to change some of the CSS.
It looks like the main .site wrapper element and the post feed wrapper elements both have a max-width set to 700px. If you have FTP or SSH access to your server, you’ll want to open up styles.css in a text editor and change .site and .site-content’s max-width to something larger, around lines 1729 (for .site) and 1748 (for .site-content) (screenshot below). There may be other instances of max-width in the stylesheet as well that you’ll need to change, but from a quick glance these appear to be the main ones.
screenshot
