The benefits of a pro account include unlimited local storage, and no maximum file size for your photos. In order to upgrade your account, you should click on your name in the upper right corner of the screen (you must be logged in first) to open the account screen.

By default your account screen should open with the first tab (account information) selected. The seventh row of your account information should list your account type and provide you with a link for upgrading.

Click the "upgrade" button to proceed.

Clicking the upgrade link should open the membership upgrade panel. This panel lists the benefits of a pro account. Click on the "upgrade now button" at the lower right to add an account upgrade to your shopping cart and proceed to the checkout.