Please follow the below steps to set up the accounting periods as desired.

1. Delete the Corporation tax returns pertaining to the period (if generated)

2. Delete the Accounts Report generated for the period (if any)
3. Delete the Trial Balance under Accounts Production>>Tasks>>Trial Balance.
4. Delete the accounting periods generated under Accounts Production>>Settings>>Accounting Periods using the action drop-down starting from the current year.
5. Once done, you may generate the accounting periods and trial balance for the desired period.

Click here to watch how to add a prior accounting period

Click here to watch how to delete an accounting period