I believe that closing the books is the option you are looking for.

This is the information for doing that:

Closing the books normally refers to the accounting process of zeroing out your Income and Expense accounts and recording the company’s Net Profit or Loss onto the Balance Sheet.. At the end […]

Resolution for 'accidental issue: 'How do I lock the previous year financials so accidental entries cannot be made to it?' available: Yes.

The accidental 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 accidental issue yourself by looking for a resolution described in the article. If it is a complex accidental issue or you are unable to solve the issue, you may contact us by clicking here or by using other accidental support options.

For more information about resolving issues related to accidental in Quickbooks/QBO or to get support for the issue 'How do I lock the previous year financials so accidental entries cannot be made to it?', click the link below: