"Please send your cancellation as a response email to your order confirmation".
If you can't do that, read §3 in the linked General terms, especially this
"You have the right to withdraw from this contract within fourteen days without stating a reason. The withdrawal period is fourteen days starting with the conclusion of the contract. To enact your right of withdrawal you must contact us at MAGIX Software GmbH, Customer Care, Borsigstr. 24, 32312 Lübbecke, Germany, Fax: +49 (0)5741 310768, Telephone: +49 (0)5741 345531, Email: shop@magix.net and provide explicit notification (e.g. a letter sent by mail, fax or email) of your decision to withdraw from the contract. You may use the attached cancellation form for this if you so choose. To enact your right to withdrawal it is sufficient to send the required notification before the withdrawal period has expired." Please read the whole text there as well.