Hi I’ve marked an invoice as paid but it hasn’t been can I change this somehow?

I can help you change invoice status in QuickBooks, @sgagri.

You can reopen a closed invoice by editing the payment and then deleting the payment that was not supposed to be made. Here's how:

1. Select the Sales menu/. 
2. Navigate the Invoices tab.
3. Locat the invoice then click the drop-down from the Actions column.
4. Select View/Edit.
5. Below the Payment Status, click the payment then open it.
6. Click on More then select Delete.
7. Hit Yes.

Here are some visual guides for your convenience:

I'm also adding this guide about managing sales forms in QuickBooks: Customise invoices, estimates, and sales receipts in QuickBooks Online.

Please let me know if you have other concerns about this matter or with QuickBooks. I'm always ready to back you up. Have a safe day!