Can I update the product/service for an invoice that has already been paid?

Hi there, daren1.

Good to see you here in QuickBooks Community. I'm here to help and guide you on how to update the invoice in QuickBooks Online (QBO).

Yes! You can update the Product/Service for an invoice that has been paid as long as the amount is the same. To do this, let me walk you through the steps: 

1. From the left menu, select Sales.
2. Choose Invoices.
3. Find the paid invoice transaction.
4. Make some update on the product/service.
5. Click Save to apply the changes. 5

If you need further assistance with the steps, I recommend calling our QuickBooks Online Support Team. They have additional tools to pull up your account and do a remote session. 

Here's how you can contact our phone support:

1. Go to: Get help with QuickBooks Online.
2. Click the contact us link. 
3. Select the Search for something else, and type-in Customer Support. 
4. Click Search.
5. Scroll-down and click Get Phone Number. 

That should do it! Please know that I'm just a post away if you have any other questions about Invoice. Wishing you and your business continued success.