Click "My Account," located just below the search box.
Type your library card number and pin/password to log in to your library account.
Click the "Items Out" tab across the top of the screen.
In the far right column you'll see "Renewals Left." As long as it doesn't have a "0" in that column, you can renew your item.
To do so, click the check box on the far left next to the item you want to renew.