You'll have to either undo the last reconciliation in the bank account affected to be able to correct the date or you can adjust the ending date of your next reconciliation. Just a heads up you'll have to unreconcile each transaction manually.
Here's how to unreconcile a transaction:
Click on the Accounting tab.
Click on Chart of Accounts.
Locate the bank account, and then click on View register or Account history.
Locate and select the transaction to unreconcile.
Under the check column, you will see an "R".
Keep selecting it until the box is blank to unreconcile the transactions.
Click on Save.
Repeat the same process for each transaction you've reconciled for the last period.
You can check this handy link on how to undo a bank reconciliation in QuickBooks Online: