Yes! Just log in to your account and visit the "Auto-Ship" section of your account. Then on the right hand side of the subscription you are trying to update the frequency for, click the "Edit" drop down and select "Edit Frequency or Renew Date". Choose a new frequency from the drop down and click save. Your subscription will now renew with this new frequency.
Please note: If you need to change the next ship date, do this as well. Changing the frequency will not update this to a new date.