Wait, so what is Project Online?
Project Online is web-based, and is great for:
(If you're looking for a hosted version of Project Server, this is it!)
To be clear, Project Online is NOT a web-based version of Project Professional. Project Online is an entirely separate service that offers full portfolio and project management tools on the web. It includes Project Web App, and can, depending on your subscription, also include Project Online Desktop Client, which is a subscription version of Project Professional.
Got the wrong thing?
If you need to cancel and subscribe to something else, choose Billing > Subscriptions in the Office 365 admin center, choose the subscription you want to cancel, and then click Cancel subscription in the pane on the right. Then, you can find the right product to subscribe to and add that to your existing Office 365 account.