Your Premium or Business package cannot be downgraded to a Free package. You can, however, Delete your Premium/Business account and order a new Free package for it.
IMPORTANT: Make sure you make backups before deleting your account, as all of your files, settings, databases will be removed with the account.
You will be required to re-upload your websites files and your email accounts, as well as emails, will be lost in this process. Keep in mind that emails are not included in backups.