I'm happy to see you posting here in the Community, Jvtyrr.
I suggest going to the Sales page and find the paid invoice. From there, you'll have the option to delete the linked payment to the entry. This way, you'll be able to change an invoice from paid to unpaid.
Here are the steps:
Log into your QuickBooks account.
On the left navigation bar, click Sales.
Go to the Invoices tab.
Find the invoice and double click it to expand the page.
Below the PAID status, click payment made and choosethe date link.
You'll be routed in the Receive Payment page. Then, click More and choose Delete.
You'll be prompted with This transaction is linked to others. Are you sure you want to delete it? Click Yes to confirm.