After logging in to your account here, under Services > My Services, click on the row where you see your domain/website. Do not click directly on your domain/website or else you will navigate to it.
You'll find your quota and shortcuts to cPanel there.
On the left sidebar, in desktop view, you can also upgrade/downgrade your account, plans, options (such as additional disk space, additional domains/websites, a dedicated IP address) and even cancel it by clicking on the appropriate button.
Note that you can't upgrade or downgrade until all invoices have been paid.