I cleared a invoice without being paid how can i undo that?

Hi there, info1271. Thanks for reaching out to the Community.
 

If an invoice was marked as paid without a payment ever being made, you can delete the transaction record to make it show correctly.
 

Here's how:
1. In your left navigation bar, go to Sales, then Customers.
2. Access the customer's profile by clicking on their display name.3. Find the invoice you're working with and open it.4. Underneath PAYMENT STATUS, you'll see a payment(s) made hyperlink. Click on it to see each one.5. Hit the payment date link to pull up it's record.
6. Select More and choose Delete.7. Press Yes.

In the event your invoice was deleted by mistake, it'll have to be recreated manually. You can use QuickBooks Online's Audit Log feature to locate its details. 

Please feel more than welcome to post a reply if there's anything else I can help with. Enjoy the rest of your day!