QB Issue Resolution:

You really can’t. Anything you enter historically will change current balances, and QBO is also date dependent so no entries allowed before you started the file, and for inventory nothing allowed before you created the item

Resolution for Issue 'QB Online = how do I roll back to previous financial year? I stupidly captured all data for FY2020 before capturing my 1st year of trading for FY2019' available: Yes (Solved).
Source: Intuit Community forum.

QB Online = how do I roll back to previous financial year? I stupidly captured all data for FY2020 before capturing my 1st year of trading for FY2019: this 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 issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

Support for financial
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