I am trying to upgrade/downgrade/cancel my subscription that I purchased on my iPhone, but I now have an Android and cannot do so from "Subscriptions and Purchases."

If you have purchased a subscription that is still active from a previous iPhone device but now have an Android, upgrading/downgrading will not be processed if you tap on "Subscriptions and Purchases" within the app. You will have to upgrade/downgrade from the App Store. You can do this from the web browser here if you no longer have your iPhone device.