Basically, you can't.
As an advocate, you do not have the system permissions to make changes to your customers' autoship orders. Only the customers themselves (or us in Customer Support) can make these types of changes.
Customers can change their own autoships with the instructions below:
1. Log in to your account
2. click auto orders
3. click cancel (blue button to the right of the autoship)
If the customer has any issues at all with this or cannot access their account, just have them contact us via the HelpDesk, sending an email to us at firstname.lastname@example.org or giving us a call at (833) 336-9223 and we are more than happy to assist!