That's annoying and we're sorry for the inconvenience.
If you've bought the wrong voucher in-store the most important thing is that you DO NOT redeem it. Once you've redeemed your voucher it can't be refunded because the credit has already has been added to your service.
As long as you haven't redeemed your voucher you'll be able to head back to the store that you bought it and ask if they will refund you. Sorry but OVO can't refund you for a voucher (or SIM card) that you bought in store.