Unreconcile journal entry first so the payment gets detached from the invoice, then you can cancel invoice and reset it to draft. then you can re-validate it again and reconcile it with payment entries so invoice gets paid again.

To unreconcile journal entry,  Open partner from invoice, in partner form there is one button "Journal Items" or "Journal Entries". By clicking on this you will be redirected to journal items (account move line) view.

There you can see all the accounting entries of that customer. You need to select two line (which lines have account receivable with debit and other with bank or cash or any other payment journals with credit). Select these two line and click on more and there is Unreconcile option.

The same way you can reconcile entries.

Hello, under the accounting point of view it would be better (and in almost every country legally correct) issues a credit note instead of deleting invoice. That is not only correct but it is also the quickest way to solve your problem.

ou need to have Developer Mode active to see the field on the Journal, then Go to "Accounting/Configuration/Accounting/Journals", select the corresponding journal and check the box under "Advanced Settings/Miscellaneous".