how do i mark an invoice as unpaid?

I can guide you with marking the invoice as unpaid, @keniccustom. 

If you already recorded an invoice payment, you can go to the invoice, open the payment that you processed, and remove the payment that you processed. 

Here are the steps: 

1. Go to the Sales menu and proceed to the Invoices page. 
2. Find and click the Edit button to open the invoice.
3. Scroll down to the Invoice Total section and click the Payment link.
4. On the Receive Payment page that opens click the Clear Payment button. 

After that or if you haven't processed a received payment transaction, you can record them as bad debt and write them off. 

You'll just have to create a bad debt item from the Products & Services tab, followed by creating a credit memo for it. Lastly, apply the credit memo to the invoice.

Here's how to create a bad debts expense account:

1. From Settings, select Chart of Accounts and click on New to create a new account.
2. Click the Account Type dropdown and click on Expenses.
3. From the Detail Type dropdown, select Bad Debts.
4. Provide all the necessary information. 
5. Select Save and Close.

To create a bad debt item:

1. From your Settings, select Products & services.
2. Click on New, and then Non-inventory.
3. In the Name field, enter Bad debts.
4. From the Income account dropdown, select Bad Debts.
5. Click on Save and Close once done.

To create a credit memo for the bad debt:

1. From the + New button, choose Credit memo.
2. In the Product/Service section, select Bad Debts.
3. Provide all the necessary information. 
4. In the Message displayed on the statement box, enter Bad Debt.
5. Click Save and Close once you're done. 

To apply the credit memo to the invoice:

1. Go to the + New button and choose Receive payment.
2. Set the customer and proceed to the Outstanding Transactions section to select the invoice.
3. From the Credits section, select the credit memo.
4. Click on Save and Close.

Here's more information about recording invoices as bad debt and writing them off: Write off bad debt in QuickBooks Online. 

Then, check out the following article for more information about recording partial and full payments: Record invoice payments in QuickBooks Online.

Let me know if you have a follow-up question about your invoices or any other QuickBooks concerns. I'm always here to assist. Keep safe!