If you're no longer in need of all the good things that come with your paid subscription to PhantomBuster, you can downgrade at any time.
To access your Billing page from your account, click on your name at the top right of your home screen and chose "Billing":
Here, you can see which plan you are subscribed to and what features are included:
Underneath your current plan you have the option to "Cancel...." You can do so at any time.
When you cancel your plan, you will continue to have full access to the resources that you paid for through the end of your subscription cycle. At the end of that cycle, you will automatically be reverted to the Free Forever plan.