All Unbounce users can modify their own account settings at any time by accessing the Account menu and going to Manage Account > Subscription > Change Plan then selecting their preferred subscription level.
If you want to cancel your subscription, access the Account menu and go to Manage Account > Subscription then click the Cancel Subscription button. This will downgrade your account to the free subscription level. On the free plan you will not be billed but will still retain access to your account and your pages.
If you want to avoid being billed at the end of your free trial, just cancel your subscription before the trial ends.