How do I change the statement date on a reconciliation?

Welcome to the Community, @nielson-vancott. It's my pleasure to help you today.

Since the reconciliation is already completed, you'll need to undo the reconciliation to correct the date. 

The Undo option is only available in QuickBooks Online Accountant. I'd suggest inviting your accountant  to your company to help you do it all at once.

Here's how:

1. Click the Gear icon, then select Manage Users.
2. Go to the Accounting firms tab, then enter the accountant's email.
3. Click the Invite button, then hit Save.

Once invited, have your accountant undo the entire reconciliation. Otherwise, you'll have to undo the reconciled transactions one at a time. 

Check out this article for your reference on adjusting the beginning balance of your reconciliation: Undo and remove transactions from reconciliations in QuickBooks Online. 

Let me know if you need more help in getting around with QuickBooks.