While using the Update a Subscription API, by default, the addons passed are appended to the existing list of addons for a subscription. In case a passed addon already exists for this subscription, quantity value is replaced. If you want to completely replace the addons for a subscription, pass replace_addon_list as "true".

Hence, if the subscription has 2 addons and you want to remove 1 addon, you would need to just pass the addon that you need to retain and define replace_addon_list parameter as "true".