Go on your Profile Page and click on the Change Now! button
You will be presented with all the possible plans, just select the one you wish to use.
In this example we are going from Premium to Base account:
You will be prompted with the payment page. This is just a confirmation request and you won't be billed again for the downgrade.
How Does This Work?
We have a system that allows you to pay only for what you are using: if your budget increases/decreases at some point during the subscription, you can change it and pay only for the remaining time that you will use with the higher subscription, or get a credit for the extra money you paid for the plan you won't use.
Can I change plan if I am already over my plan's limit?
No, you cannot. You will have to wait until the next billing cycle - however the extra credit will be stored for you on downgrade as well.