How do I cancel seventeen magazine subscription?

Okay, so I have a am suscribed to seventeen magazine and thats fine. but I accidencly suscribed another time( I was trying to enter the freebie) so I made another subscription cause i forgot to log on to my other account. how do i cancel the second subcription? It tells me I have to pay 10 dollars i dont have a magazine yet.how do I just cancel it online?