Log in on the 0x Developer Dashboard, click "downgrade" and send a message to 0x Developer Support explaining that you would like to downgrade your plan.

We will schedule your plan downgrade for the next month, so you will continue to enjoy the benefits of your current plan until the end of the current month. You will still have access to the higher quota, higher throughput, and applicable apps during this time.

Starting from the next month, you will be billed for the new lower amount, and the new quota and rate limits will apply. If you have more apps than are allowed for your new tier, you will need to delete them.