Click the section settings cog icons, either at the top-left or the bottom right to bring up the Contentbox settings popup window.
From the settings that appear, choose the Content Width from the GENERAL PROPERTIES dropdown.
Special Content Width settings
Default Width - Applies the max-width as set in the template.
Full Width - Sets the content to full width of the browser, but allows some spacing on the left and right edge.
Max Width - Sets the content to full width of the browser, no padding.