Follow these instructions to upgrade or downgrade and existing subscription.
1.) Hover over your name in the top right. This can be done on any screen. A drop-down should appear. Click on Subscription.
2.) After the Subscription page loads, click on the purple "Manage your subscription" button.
3.) Click on your subscription in the box.
4.) Click on the "Edit Subscription" link.
5.) Click the dropdown button and choose your new plan, then click the "Update Subscription" button. That's it, you're done!
NOTE: Any changes to your plan will affect you current lists. If you upgrade a plan, your current list's limits will increase automatically. Conversely, if you downgrade your plan, you will also downgrade your list's limits.