Yes! Just log in to your account and visit the "Autoship" section of your account. Then, next to "Frequency" on the product you'd like to change, click on the link. In the popup, choose a new frequency from the drop down and click submit. Your subscription will now renew with this new frequency.
Please note: If you need to change the next ship date, you'll need to do this as well (by clicking on the date as shown in the screenshot above). Changing the frequency will not update your next shipment date.