this happened to me the other week and you cant mark it as unpaid, you can resend the invoice but you have to log into paypal to do it.......... if you're not sure when you log into paypal up the top of the page is request money.....click on that and you'll see create invoice and just fill in the details and resend

if you wish to open an unpaid dispute you might have to contact paypal and they can talk you thru it