Good morning to you too, Rachel.
You can only change the first month of your financial year to September in QuickBooks Online (QBO). You're unable to select a specific year or date. Also, the effect is not retroactive especially when viewing your reports and data. It will still display the date parameters you've previously used.
What we can do is to run your reports, manually open your transactions, and modify the date. After that, you can move on to the stages in changing your financial year. Here's how to do it.
- Click on the Gear icon and select Accounts and settings.
- Select the Advanced tab and click on the Accounting section.
- Choose September for the first month of fiscal year option.

- Click Save, and then Done.
In regards to your bank transactions, you can exclude those incorrect bank statement and upload the correct ones.
Once you're all set, you might want to utilize this link for reference in efficiently matching your accounts: Reconcile an account in QuickBooks Online.
I'll be around if you have any other concerns or questions about your accounts and reports. Simply add the details in you reply and I'll get back to you as soon as possible. Have a great day ahead!