It's very easy to adjust the products in a customer's subscription in any way you might need to. Below we'll cover how to adjust the quantity, remove items, and add items.
You can adjust quantity up or down of any product in a subscription by pressing the up and down arrows in the product line under "Quantity":
You can also remove products by bumping them down to quantity 0. Any chance will have to be saved before it is put into effect. There must be at least one item in the subscription, so you will not be able to move the last item's quantity to 0.
Use the overflow menu on any product line to remove a product from the subscription. The quantity will be set to zero and you will have to save before the change goes into effect. Remember that if there is only one item in the subscription it cannot be removed - subscriptions must have at least one item in them or be cancelled.
You can add products by pressing the "Add product" button below the current list of products in a subscription:
This button will open a list of all the products in your store. You can add them regardless of whether or not they are part of a subscription plan, but they will be added on a recurring basis.
Once you select a product (or products) to add, you will be asked if you want a discount to be applied. You can choose a certain amount off or a percentage, or set it to 0 to keep the original price of the item.
Once you've confirmed, you'll see the new item show up in the list for the user's subscription.