To upgrade your card plan, go to card plans (make sure you are signed in), and then click on the upgrade button next to the new plan that you would like. You will only pay the difference between your current plan and the new plan, and you will keep your same renewal date.
For instance, if you purchased the "Box" plan (10 cards) six months ago and would like to upgrade to the "Drawer" plan (25 cards), you would pay the difference between those two plans and you would receive 15 additional card credits. Your card plan would renew in six months at the "Drawer" level. If you would like to change your renewal plan, see How do I change my renewal plan? (article coming soon).