When you purchased, there were instructions as to what to do to cancel. You should have read them and noted them.
Now, you can go back to where you purchased on Magix.com, PhotoStory, Subscription and click on the "i" and read what it says. The instructions to cancel are indicated there.
Email contract number (login to Magix, it's under My Contracts) and request to end the subscription to infoservice@magix.net at least a month before contract end. As for Magix, they could have added this info to one's account under My Contracts section rather than making it difficult and confusing.