How to change paid invoice to outstanding again

I've got steps to ensure you can change the invoice status from paid to outstanding, @roig.

The following are the quick and easy steps on how to achieve this in QuickBooks Online (QBO):

1. Go to the Sales menu at the left pane, and then choose the Invoices tab.
   
   
2. Look for the invoice you want to modify the status from paid to outstanding, and then click the payment link.
   
   
3. Ensure to check the payment made in the Outstanding Transactions section, and then tap Clear Payment.
4. Click Save and Close once ready.
   
   

You'll want to customize your invoices to personalize their info and formats. Also, you may email or print them in bulk to make your task easy and efficient.

To keep track of all your unpaid invoices, you can run the Open Invoices report in QBO. Simply explore the Reports menu, and then refer to the Who owes you section under the Standard tab. Please see screenshot below for visual guide:

I'm always around to lend a hand if you have more QuickBooks concerns. Take care and have a productive day.