I accidentally did mark the invoice as paid which in not paid. How do I fix this?

Welcome to the Community, Ensoni. I've got the steps to remove the paid status on your invoice in QuickBooks Online (QBO).

1. Go to Sales, then choose All sales.
2. Locate the payment.
3. Tick the dropdown and choose Delete.
4. The Paid status will now be removed.

Additionally, you can refer to this article for guidance on personalizing your sales forms: Customise invoices, estimates, and sales receipts in QuickBooks Online.

Comment on this thread if you have further concerns with your invoices. The Community is available 24/7 to assist. Have a great day!