There are two ways to convert your account, depending on whether you have an active subscription or are still using a trial account.
If your account is in trial
If you need to convert from a Pro account back down to a Basic account then you can do so using the link found on the Subscription page. This can be found in the Account Settings of an account that currently has no subscription. Below the list of stacks you will find a Convert to Basic Account link as shown below:
If your account has an active subscription
If the account you wish to convert to basic already has an active subscription then the Subscribe link mentioned previously will be missing. Instead, you will be able to view your current plan on the Plan page which can be found in Account Settings.
If you wish to convert your account then please contact support. We would be happy to help.