You go into "Settings" at the top right corner of the page. On the next screen, scroll down until you see a box titled "My Settings". In that box you need to click on General Settings under My Account. When you get to the next screen, scroll down until you see the Thread Display Options section. Then look for the subheading Thread Display Mode. To change the setting, click on the arrow in the corner of the little box and change the setting to "Linear-Newest First".