That Purchase Req. is linked to PO (in fact, vice versa - PO is linked to PR), both are completed transactions, you have later “rolled back” the PO with a CreditMemo or Return Order when you found out there was something wrong - now all that is history.
Just start again with new correct Purch.Requisition if company policy demands such workflow. (or you may create the correct PO directly, IF exceptions from standart workflow are allowed).