To add items: go to the product page of the item you'd like to add and click the "Add to Autoship" or "Ship once with next Autoship" button. If you have multiple Autoship orders, you'll be asked to select which one you're adding to!
To remove items: go to Manage Autoship and select your Autoship order. For the item you wish to remove, click on "Remove" and confirm you'd like to remove it from your Autoship order.