I made a mistake. have mad an invoice as paid but it was the wrong one. how mark this invoice as unpaid?

Hi there, YepYep. I'll help you with correcting your invoice transaction in QuickBooks Online (QBO).

At this time, you need to delete or remove the payment transaction to unlink it to your invoice. Let me show you how.

1. Sign in to your QBO account.
2. Open the payment transaction.
3. Tap on the More tab, then select Delete.
4. Hit Yes to confirm the action.

I'm adding this link for more tips about deleting or voiding your QuickBooks transactions: Void or delete transactions in QuickBooks Online.

On the other hand, here's how you can get more ideas about handling your sales and other customer transactions: Topics about your company's income and customers.

If you need additional assistance with marking your invoice as unpaid, let me know by adding a comment below. I'm more than happy to provide additional assistance. Cheers for more success!