How do i change an invoice from paid to still due

I got the steps you need to achieve this, @lindak1119. and I’d be more than happy to guide you how.

From your QuickBooks Online (QBO) account, let’s go to the Customer tab and view your customer transaction list. There, locate the payment transaction of the invoice and delete it. Let me show you how:

1. Go to the Sales menu.
2. Select the Customers tab.
3. Click the appropriate customer name.
4. From the Transaction List column, locate and open the Payment transaction.
5. Uncheck the Invoice to unlink the payment and click the Clear Payment button to avoid creating an open credit. 
6. Then click Save and close.
7. Go back to your customer transaction list and you’ll see the status of the invoice is Open.
8. Click the Invoices tab and you’ll see the status of the invoice is still due. 

I’m also adding this article that you can use for reference when recording your customer payments: Record invoice payments in QuickBooks Online.

Additionally, in case your customer pays you upfront, you'll want to use a sales receipt to record the payment. Here’s an article you can read for guidance: Create and send a sales receipt.

Let me know if you have other follow-up questions regarding invoices in your account. I’ll be around to assist you. Keep safe and take care!