You can easily manage how often you'd like to receive your subscription by visiting your My Account page. Just look for the Subscriptions section.
Under Subscriptions, you can select Edit on the product you'd like to modify. Here you can modify the quantity of product you are receiving, and how often you'd like to receive it. You can select from 2, 4, 6, or 8 weeks.
Once you've updated, be sure to select "Save Changes