There is no way to downgrade a subscription, only upgrade.  You will have to cancel your current subscription and start all over with a new one.

Resolution for 'business version issue: 'I am trying to change my subscription. I cannot see how I can change from QB Plus to the basic small business version.' available: Yes.

The business version issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the business version issue yourself by looking for a resolution described in the article. If it is a complex business version issue or you are unable to solve the issue, you may contact us by clicking here or by using other business version support options.

For more information about resolving issues related to business version in Quickbooks/QBO or to get support for the issue 'I am trying to change my subscription. I cannot see how I can change from QB Plus to the basic small business version.', click the link below: