Thank you for connecting with us here in the Community, @nsaroli. I'd be happy to help you sort things out.
To void a check from a closed period, enter a deposit for the total amount in the current period and post it to Other Liability. Also, include the Check numbers you are voiding in the description. This deposit will be used to offset the amount of all unpaid checks.
When you reconcile the bank account, make sure the checks have been voided and the deposit has been cleared.
Then, you can delete the journal entry you've created. And before making any changes to your prior period, I recommend consulting with your accountant to ensure the accuracy of your books.
You can visit this link to know how to run different types of reports in QuickBooks Online: Run Reports In QuickBooks Online.
Feel free to reach us back if you need further help with your transactions. Have a good one!