Ho do I undo an allocated payment. By mistake allocated payment received to the wrong invoice

Let me help you fix this out, @payments-shiftde.

If you accidentally match the payment to the wrong invoice, you can still undo it. Just go to the Banking page and unmatch it. Here's how:
 

1. Go to the Banking menu.
2. Choose tile for the bank or credit card account you downloaded the transaction.
3. Click the Categorised tab.
4. Put a checkmark on the transactions you want to unmatch
5. Lastly, click the Undo option.

Once done, you can now match the payment to the actual invoice. 

I've also added this resource for more insights: Unmatch downloaded bank transactions or move them to another account.

Feel free to comment down below if you have any other QuickBooks concerns. I'll be more than happy to help you. Have a wonderful day.