You must cancel before the end of 6 months to avoid any charges, upgrade to paid membership is automatic.
Amazon is offering free Amazon Prime for 6 months for college students. Once you have been verified through an .edu email address, you can enjoy this promotion for Free Two-Day Shipping on millions of items, and many other deals.
At the end of your free 6 months, your membership will automatically upgrade to an annual Amazon Prime membership and you will be charged $49/year. You may cancel anytime by visiting Your Account and adjusting your membership settings. You will continue to receive Amazon Prime for $49/year for as long as you are an Amazon Student member for up to 4 years (free and discounted) after which your Amazon Prime benefits will renew at full price of $99/year.