How do you cancel ur trial in Seventeen without canceling ur paid sub?

I have a one year subscripition that is already paid for. I went online to enter a freebie contest but as I submitted the form, I forgot to uncheck the box for their Seventeen trial offer. Few weeks later I waited for my magazine but nvr came. BUT, a bill came and said I hve to pay $10. So I went online to cancel the trial but I can't figure out how to cancel without canceling my sub. Help?