Payment method for what??? Since you've chosen MM as your product I'm going to guess this is for Unlimited Loops. Is that correct?
If so, I think you need to access your Magix account, log in and go to My Contracts where I suspect you can make the change you want to do. If that is not successful, you may need to contact sales@magix.net and ask for their help in this.
Actually I went to My Contracts but since I don't have any subscriptions the option to change payment methods did not appear. I want to change the credit card I use for payment.